The Global Positioning System (GPS) ray-shooting method is a self-sufficient observation operator in GPS/MET (Meteorology) data variational assimilation.The huge computation of the GPS ray-shooting method,however,makes it impossible so far to be applied in data assimilation and operational prediction.In order to reduce the huge computation,a 2nd-order timesaving symplectic scheme is used to solve the equations of the GPS ray trajectory due to its separable Hamiltonian nature.Not only does it save 75% of the CPU time the old GPS ray-shooting model using 4th order Runge Kutta method takes,but also does the simulation accuracy be improved slightly to some extent.Then the parallel computing of the new GPS ray shooting model is studied.The speed up and the parallel efficiency are both very high.