The design constrained equations of azimuth drift is a nonlinear one that contains 3 independent and several hidden unknowns, the numerical iteration method should be used to get the numerical solution. The vertical depth increasing equation was provided, by using vertical depth increasing equation of the constrained equations, one of the 3 independent unknowns was expressed as functions of the other 2 unknowns, which were used for dimension reduction of the design constraint equations. The paper analyzed the calculation details of hidden unknowns and gave the recursive algorithm for hidden unknowns. The numerical value arithmetic of constrained equations after dimension reduction was also put forward, that was half- shrinkage grid method, which could be applied to quickly and reliably find the numerical solutions of design, especially for programming computer software.