The invention discloses a digital control method for a DC (Direct Current)-DC converter based on LQR (Linear Quadratic Regulator) optimum control. The digital control method comprises manual/automatic control mode switching design, state observer design, an LQR optimal controller with integral control and considering of tracking performance of reference input voltage, and the design of a method for resisting integral saturation. According to the digital control method disclosed by the invention, the LQR optimal controller with the integral control is designed, the output residual error of the DC-DC converter is effectively eliminated, and the tracking performance of the reference input voltage is considered, so that output voltage of the DC-DC converter can be used for better tracking the change of the reference input voltage in real time; the method for resisting the integral saturation is adopted for processing the control variable duty cycle output, and the problem that the controller is failed as the duty cycle is saturated for a long time caused by wide-range change of loads and the like in the actual engineering application is avoided; a switch is adopted for controlling the manual/automatic control mode to be switched, so that smooth switching of two operating modes is guaranteed. Shown by the actual engineering application, the digital control method disclosed by the invention has good control precision and robustness.