The formation mechanism of P-Δ effect of bridge double-pile foundation in steep slope was analyzed. According to the theory of the earth pressure and the elastic foundation beam, a pile-soil interaction model was established on the basis of the reasonably simplified residual pushing force and soil resistance, and then the differential equation of the characteristic section of piles was deduced with consideration of the P-Δ effect, and the solution to the calculation of the inner-force of the double-pile was obtained by using the power-progression method. In addition, a comparative analysis of an existing literature was carried out, and it has been found that the calculated values are closed to the measured values. Finally, the influencing factors and sensitivity of inner force and the displacement of the bridge double-pile were studied and some useful conclusions have been drawn: the P-Δ effect of the former-pile can not be ignored; the moment of the back-pile is bigger than the former-pile when the load level and the soil resistance coefficient m are the same, and the maximum bending moment surfaces of the pile is near the sliding surface. The effect of embedded depth also exists in the piles in steep slope.