A novel design of the gas Cherenkov detector (GCD) using three pieces of 90° off-axis parabolic mirror and a turning flat mirror will take place the conventional GCD using Cassegrain reflector optics. Light collection is more effective by the using of the off-axis optical system and positioning the optical detector at the exit pupil of the optical system. Optical ray tracing demonstrates that how light can be collected by the optical detector from different object surface. An integration design of the whole GCD is given and the details are analyzed to enhance light collection efficiency and offer better radiation shield.