Abstract: The secular influences of the variable magnetic radiation braking torque on the spin down of two-components of pulsar are studied by using analytical method. The analytical solutions of the coupled equation system are given with due consideration of the time-dependent magnetic braking torque by solving the system of non-homogeneous equations. The analytical solutions obtained are applied to the research for PSR0531+21 (Crab Pulsar). The numerical results are obtained in Table 2. The numerical results show that the spin of Crab pulsar speeds down -0.245rad/yr. Discussions are held on the obtained results.