The invention discloses an inertial system coarse alignment calculating method based on an OPREQ method. The calculation of an initial attitude matrix is transmitted to determination of a constant attitude matrix, and the OPREQ method is employed for solving an optimal quaternion corresponding to the attitude matrix. The gain size is subjected to adaptive adjusting, noise in an observation vectoris effectively subjected to filtering processing, so that the effect of coarse alignment convergence performance can be achieved, and the coarse alignment method of a static pedestal and a swing pedestal has good engineering reference and application value.