We present the final film thickness' expression of spin-coated photoresist on a spherical substrate. Firstly, some reasonable assumptions are put forward for a concise derivation process. Then, on the basis of the motion equation of spin-coated photoresist on a plane, considering the spherical surface shape, we put forward the motion equation of spin-coated photoresist on a spherical substrate. So two evolution equations of film thickness and radial position are derived, and the expression of initial film thickness evolution in a radial position is also gained. Finally, considering some effects of solvent volatilization, we gain the expression of final film thickness. The experiment result indicates that the expression is accurate.