Abstract: In the pile foundation,when the pile group by is under horizontal force, if the pile group spacing is less than 8 times the pile diameter, pile group horizontal force is not equal to the sum of horizontal force of each pile, through test get smaller value of each pile horizontal force are often used in previous studies. P factor is a typical method of pile groups capacity reaction, but the theoretical research in this area is relatively small, the paper summarizes and analyzes the previous experimental data,makes a summary to the P factor, gets a theoretical formula, the study plays a catalytic role in the horizontal force of pile group.