To discuss the seismic behavior of RPC box piers, a nonlinear analysis program was compiled with the fiber element model by taking into account secondary effects of the axial force, in which the horizontal load under variable loading direction could be simulated and the whole loading process including the unloading could be analyzed. Based on the contrast analysis between cyclic loading test results of three RPC box piers samples under the invariable axial force and the calculation results, the precision of the program was proved. Factors affecting the seismic behavior of RPC box piers under the variable horizontal loading direction were analyzed, including axial compression ratio, reinforcement ratio, length-width ratio and opening ratio of the section, and the relationship between the most unfavorable loading direction and length-width ratio of the section was obtained.