In this work we report on calculation of SHG intensity in a nonlinear film on top of Fabry-Perot cavity. The system is of particular importance, because it represents wide and intensively studied 2D semiconductors on top of  substrate. Calculations of SHG intensity are performed using optical matrix technique [1] and result in an elegant and clear expression which allows direct substitution of optical constants, which is definitely appreciated by experimentalists.     Since nonlinear optical processes require high laser power density, strong light-matter interaction may change optical constants. Our estimations based on the dependence of the SHG intensity on the thickness of  layers show that if the photon energy falls away from the band edge, optical Kerr effects play the dominant role in the optical constant modifications. If the pump energy approaches the band edge and excitonic resonances, then additional effects of free carrier generation may become important.   Fig.1. Dependence of the SHG intensity from the thickness of the flake. Points: experimental values. Lines: theoretical dependences One more important result is the proof of different conditions for the best observation (visualization) of TMD attached to  substrate in linear and SHG images. The obtained result is also important for calculations of nonlinear optical susceptibility of TMDs attached to  Fabry-Perot cavity.