[Objective] The material properties of High Density Polyethylene (HDPE) pipes, such as elastic modulus and Poisson’s ratio, differ significantly from those of cast iron and PVC pipes. Considering the unclear influence of branch parameters on water hammer in branched pipelines, this study aims to investigate the water hammer characteristics in branched water supply HDPE pipelines using a systematic experimental approach. [Methods] Experimental studies were conducted to examine the effects of four individual factors—the bifurcation angle θ, the diameter ratio D between branch and main pipes, the location of the bifurcation point L, and the length ratio L_d between branch and main pipes—under different operating conditions of the main and branch pipes. Specifically, the conditions include: main pipe operation, where only the main pipe conveys flow while the branch pipe is closed; branch pipe operation, where only the branch pipe conveys flow while the main pipe is closed; simultaneous operation of both pipes, where both the main and branch pipes convey flow concurrently. [Results] The characteristics of water hammer during valve closure under simultaneous operation of the main and branch pipes differ from those under independent operation.(1) Under independent operation, variations in branch pipe parameters have minimal impact on the water hammer fluctuations and pressure rise in the main pipe. The pressure rise in the branch pipe increases only with an increase in L, while remaining unaffected by changes in the bifurcation angle θ, the diameter ratio D of branch to main pipes, or the length ratio L_d of branch to main pipes.(2) During simultaneous operation of the main and branch pipes, water hammer waves superimpose within the system, with the most pronounced superposition effect observed at the main pipe valve. The pressure peaks in the main and branch pipes reach 1.12-1.73 times and 1.27-1.66 times, respectively, of those under independent operation.(3) The influence of different branch pipe parameters on water hammer superposition varies: θ and L_d have a relatively minor effect on pressure peak values, their occurrence locations, and the pressure distribution along the pipelines. The pressure peak decreases as D decreases, with the maximum pressure peaks in both main and branch pipes occurring at D=1.0. The water hammer pressure peak in the main pipe decreases with an increase in L.(4) The viscoelastic Joukowsky water hammer pressure calculation formula provides suitable accuracy for estimating direct water hammer pressure in main and branch HDPE pipes when they operate independently. This confirms its utility in simpler scenarios. However, the application to calculate direct water hammer pressure during valve closure under the complex simultaneous operation regime reveals a significant shortcoming consistently yields results that are lower than the empirically measured values, with substantial relative errors ranging from 22.83% to 87.16%. These systematic discrepancies clearly demonstrate that branch pipe parameters exert a significant and non-negligible influence on the water hammer characteristics during simultaneous valve closure in bifurcated water conveyance systems. Consequently, the standard viscoelastic Joukowsky water hammer calculation formula requires substantive modification and adaptation to accurately model the interactive dynamics present in HDPE bifurcated pipelines under such simultaneous operational conditions. [Conclusions] During the evaluation and selection of pipeline design schemes involving branched pipelines in engineering practice, the following recommendations are proposed: a branch pipe with a relatively smaller diameter and a larger branch-to-main pipe length ratio should be selected, and the branch pipe bifurcation junction should be positioned closer to the upstream section of the main pipe.