Based on waveguiding mode theory and non paraxial vectorial moment theory, the beam quality of TE 00 mode propagating in GaAs/AlxGa1-x As strip buried heterostructure semiconductor laser is investigated theoretically, and the emphasis is laid on the variational relation between the beam quality factor in the direction parallel to the junction plane M2y and the active layer width W. The results show that M2y fluctuates with the active layer width W, which is useful for the proper design of the active layer width.