The previous researches show that appropriate power modulation methods can increase the weld penetration and decrease the drawbacks of splatters and pores in laser welding for the high reflectivity materials such as magnesium alloy, aluminium alloy and copper alloy. This paper conducts tentative explorations to the underlying physical mechanism of this phenomenon. The sine modulated laser welding test of AZ31 is developed based on ternary quadratic regression design and the influence of average power (AP), modulation amplitude (A) and frequency (F) on welding joint cross-sectional fusion zone area (FZA) are studied. The results show: power modulation can obviously improve energy coupling in the lower AP welding process, while this improvement will be weakened and even disappeared with AP increasing. Both “Small amplitude + high frequency” and “large amplitude low frequency” can increase the cross-sectional FZA when welding in the case of lower AP. The longitudinal-sectional morphology of “stainless steel (reflectivity: 60%) Mg alloy (reflectivity: 80%) ” bimetallic specimen welded joint obtained in the sine modulated welding of 8Hz is achieved and compared, which demonstrate when power begin to decrease from the peak value, the weld depth in lower reflectivity materials 2205 is decreasing synchronously, but the decreasing moment of weld depth in high reflectivity materials AZ31 delays about 0.036s(about 30% of a sine period).The reason for this phenomenon may be that a deeper keyhole could be formed by the instantaneous peak power value, which increase the number of reflection of laser beam in keyhole. Then, in spite of the declining of transient laser power, the high aspect ratio keyhole will be maintained for about 1/3 of a sine period due to the enhanced energy coupling efficiency. It is the key point for power modulation to improve the energy coupling efficiency of high reflectivity materials that the high aspect ratio keyhole can be existed and maintained for a long time.