In this paper, we present the preliminary optical system design of AST3-NIR camera, a wide-field infrared imager for 50cm Antarctic surveying telescope (AST3-3) to be deployed to Dome A, the Antarctic plateau. It is a joint project in which China is responsible for telescope hardware and control, logistics and deployment. Australia is responsible for instrument hardware design and control, and power generation. The camera uses two mosaic Leonardo detectors with 1280 x 1032 pixels each. The instrument is designed with a field of view(FOV) of 28.10 X 46.10 at the pixel scale of 1.35" per 15μm pixel. It is optimized for K dark band (2.26μm to 2.49 μm). The main challenges of this design are to produce a well-defined internal pupil stop located within cryogenic condition which reduces the thermal background and the correction of off-axis aberrations due to the large available field. Since the operating temperature of the camera could vary from -35°C to -90°C, the refocusing mechanism needs to be designed within the camera. The optical performance of the system will be demonstrated. We show the opto-mechanical error budget and compensation strategy that allows the built design to meet the optical performance.