Bases on the analysis of the equivalent circuit model and the commutation process of the BLDCM, the effects of electromagnetic time constant and the back EMF to torque ripple were considered, by solving the time what opening and closing phase need, the relationship between the two different operating conditions was analyzed, introducing a variable which called turnon angle, controlling the torque ripple during the phase change by controlling the opening time of the opening phase; and the non ideal air gap magnetic density and other factors to modify the opening angle were considered. Combined with finite element simulation, the results showed that this method was feasible.