To solve the problems in current plane gate structures such as uneven distribution of internal forces and large bending moment in main girder mid-span, a new main girder structure of plane steel gate fixed by elastic bearing is proposed according to the theory that the bearing capacity of statically indeterminate structure is higher than that of statically determinate structure. Taking the equal safety degree between mid-span section and bearing section as a principle, the reasonable corner of the elastic fixed bearing section is determined ； and based on structural reliability theory, the reliability index of the main girder is evaluated and is compared with the reliability of existing structure girder. The results show that the new structure can improve the reliability index by 1.44 under a constant main beam section;or reduce the material consumption by 30% under a constant reliability index ； and the distribution of reliability along the span length is uniform. Therefore, it is an economical, safe and reliable structure.