Membrane or film gain mediums have attracted great attention for their good thermal performances and light field manipulation properties, which are helpful for the power scaling of a laser. A membrane laser enabled on the waveguide grating structure is proposed and this design is flexible to manipulate optical properties by controlling the photonic density of states. Lasing behavior is theoretically analyzed based on the four-level rate equations. Results show that pump absorption is enhanced by about 35 times, which leads to improved laser efficiency. When the laser mode overlaps the resonant guided mode, the Purcell factor achieves 212, leading to enhanced emission rate, as well as an apparent decrease of relaxation oscillating amplitude and time delay. The present work offers a preliminary design and lasing behavior analysis of the vertical external surface-emission laser enabled on the waveguide grating structure.