Controlling nonlinear light-matter interaction is important from fundamental science point of view as well as a basis for future optoelectronic devices1,2. Recent advances in two-dimensional crystals have created opportunities to manipulate nonlinear processes electrically3-8. Here we report a strong second-harmonic generation (SHG) in a 2D WSe2 bilayer crystal caused by a back gate field. This unusual process takes place only when the gate polarity causes charge accumulation rather than depletion. Analysis based on bond-charge model traces the origin of SHG to the non-uniform field distribution within a single monolayer, caused by the accumulated sub-monolayer screening charge in the tungsten plane. We name this phenomenon charge-induced SHG (CHISHG), which is fundamentally different from the field- or current-induced SHG3-11. Our findings provide a potentially valuable technique for understanding and noninvasive probing of charge and current distributions in future low dimensional electronic devices.