For the sake of the requirement for attitude determination at arc second order in spacetoearth platform, the imaging model of star sensor in inertial frame is constructed. It is pointed out that the main error for attitude determination of star sensor is the positional error of star in focal plane. Meanwhile, based on the defocused model of ideal optical system, the quantitative relationships among the main indexes of star sensor are analyzed and the minimum projection position error is also given. It is shown from the simulation results that, the blur area's diameter between 2～3 pixels is the best performance scheme using the slightdefocusing of star sensor's focal plane array and the positional error is only 0.2 pixel, which meets the requirement of star sensor about 4～5 arc second in inertial frame. The analyzed and simulated results can be references for the following indexes determination and designing of star sensor.