The present invention discloses an image sensor circuit and a ramp signal generator thereof. The image sensor circuit includes: an active pixel sensor (APS) array which includes plural pixel circuits arranged in an array of columns and rows; plural slope analog-to-digital converter (ADC), wherein each of the slope ADC is coupled to the corresponding column, and generates a digital sampling signal according to a ramp signal together with a pixel signal including a reset signal and a image signal which are generated by the pixel circuit located in the selected row and in the column corresponding to the slope ADC; and the ramp signal generator, which generates the ramp signal, wherein the ramp signal generator includes an active integrator, and the active integrator generates the ramp signal by charging an integration capacitor with a gain current.