The invention discloses a hydraulic cylinder displacement digital servo control system, the system comprises a digital control computer, and a digital-to-analog conversion module, a servo amplifier, a servo object, a hydraulic source, an optical-electricity encoder and a high-speed counter module; the servo object is formed by connecting the output end of an electro-hydraulic servo valve, the hydraulic cylinder and a mechanical load in series, the output end of the digital control computer is connected with the digital-to-analog conversion module and the servo amplifier in series and is connected with the input end of the electro-hydraulic servo valve, the electro-hydraulic servo valve is also connected with the hydraulic source, and the output end of the mechanical load is connected with the optical-electricity encoder and the high-speed counter module in series in sequence and is connected with the input end of the digital control computer. According to the invention, the mounting is simple, the cost is saved, the system is fast in response and is non-overshooting, and the system has extremely strong anti-disturbance performance. The invention also discloses a hydraulic cylinder displacement digital servo control method.