[Objectives] In recent years, intense channel evolution in the lower reaches of Minjiang River has negatively impacted channel stability, flood control, water supply, and aquatic ecosystems, thereby constraining socio-economic development in riverside cities. The mainstream of the lower Minjiang River (from Shuikou Dam to Huai’an Diversion Outlet) has been a key sand mining area, with significant riverbed incision. However, previous studies have paid insufficient attention to this phenomenon. This study aims to thoroughly investigate the evolutionary processes and characteristics of riverbed incision in this reach and quantify the impact of sand mining on the riverbed incision. [Methods] Using measured topographic data from seven different years (1999-2020), the evolutionary characteristics of riverbed incision were analyzed in terms of planform changes, cross-sectional profiles, and scouring and deposition variations. Based on river sand mining data, the contribution ratio of sand mining to the riverbed incision was calculated. [Results] (1) the mainstream of the lower Minjiang River experienced intense overall scouring from 1999 to 2020, with scouring erosion occurring across 84.18% of the channel area, resulting in a total scouring volume of approximately 255 million m³. The average incision depth was 5.09 m, while the thalweg exhibited an average incision of 7.87 m, with a maximum incision depth approaching 20 m. (2) The riverbed underwent rapid scouring before 2011, whereas the scouring rate significantly decelerated thereafter. The total scouring volume during 1999-2011 and the average thalweg incision accounted for 82.51% and 76.11% of the respective totals for 1999-2020. (3) Except for a slight overall deposition in the riverbed during 2014-2017, net scouring occurred in all other periods, with the highest mean annual incision rate observed between 2008 and 2011. (4) The most pronounced incision occurred in the 15-30 km reach downstream of Shuikou Dam, where the average incision depth reached 6.92 m during 1999-2020, and significant incision persisted there after 2011. (5) Sand mining was identified as the primary driver of riverbed incision, estimated to contribute over 50% to the riverbed incision. [Conclusions] The findings reveal the processes, characteristics, and trends of riverbed incision in the mainstream of the lower Minjiang River in recent years and identify the most severely incised reaches. For the first time, a quantitative assessment of the impact of sand mining on the riverbed incision is provided, and it is suggested that sand mining has been the dominant factor driving riverbed incision in the mainstream of the lower reaches of Minjiang River in recent years. These results provide fundamental support for channel evolution, river management and sand mining planning, and also offer valuable references for flood control, river channel protection, and river-related project development.