The invention discloses a method for determining a parallel development step pitch of inner and outer dumping sites of soft rock opencast coal mine, and belongs to the technical field of open pit mining, and the method comprises the steps: firstly, determining the safety reserve coefficient of a tall and large composite slope formed by an outer dumping site, a stope and an inner dumping site independently or by combining two or three of the outer dumping site, the stope and the inner dumping site according to the service life of the slope; according to the shape of the side slope, adopting a bishop method and a residual thrust method to calculate and obtain the stability coefficient of the side slope independently, or the stability coefficient of the side slope combining the two or the three of the outer dumping site, the stope and the inner dumping site, comparing the error between the stability coefficient and the safety reserve coefficient, and adjusting the flat plate width of the outer refuse dump, the distance between the top of the stope and the bottom of the outer dumping site and the flat plate width of the innerdumping site; and based on the optimal value of the values, adjusting the forward development step pitch of the outer dumping site. According to the method, the space of the inner and outer dumping sites is quickly and fully occupied, the stripping distance can be shortened, the mining cost of the open pit coal mine is reduced, and the defects of lagging development and large distance of the traditional design outer dumping site are overcome.