The structural parameters of butterfly multimode interference (MMI) coupler need to be determined according to the special ratio of power splitting when the device is designed. The coupling length, as a characteristic parameter of rectangular MMI coupler, is determined by the finite difference beam propagation method (FD-BPM) which is applied to the modeling of symmetrical interference rectangular MMI coupler and then the structural parameters of butterfly MMI coupler are designed by the formulas of mode propagation analyse (MPA) method which is an approximate method. Finally the FD-BPM is used to correct the parameters of butterfly MMI couplers, including the structural parameters and the ratio of power splitting. It is indicated by an example of practical device in SOI wafer that the length of butterfly MMI coupler obtained by simulation is more than the length expected by MPA theory within the range of two to four micrometers. The actual power splitting ratio is lower than the predicted values of MPA theory. And the further the shape of butterfly MMI coupler departures from the shape of rectangular MMI coupler, the more deviation grows between these two values.