Cavity quantum electrodynamics dictates that the spontaneous emission rate of a nanoscale light emitter can be significantly enhanced through engineering its electromagnetic environment by a photonic nanocavity in Purcell regime. We have observed such Purcell effect by integrating a monolayer semiconductor WSe2 with a photonic crystal cavity (PhCC) recently. In principle, this effect can greatly reduce the lasing threshold of the emitter, providing the ultimate low threshold laser with small footprint, low power consumption and ultrafast modulation. In this project, we employ such principle to achieve a new kind of nanolaser, where the atomically thin crystalline WSe2 is nondestructively and deterministically introduced as the gain medium at the surface of a prefabricated PhCC (Q ~ 8000). A continuous-wave operation is obtained in the visible region with an optical pumping threshold as low as 27 nW at 130 K. Compared to the conventional PhCC laser design, the surface-gain geometry is a new strategy and allows unprecedented accessibilities to multi-functionalize the gain, not only enabling the electrically pumped operation but also highly compatible to CMOS and silicon technologies. Such scheme is therefore highly scalable and compatible with integrated photonics for on-chip optical communication technologies. Along this demonstration, the strong coupling region in Cavity-QED, photonic quantum control and nonlinear photonic based on 2D semiconductors and their heterostructures are anticipated.