This paper systematically establishes a range resolution model for 3D gated range-intensity correlation imaging (GRICI) based on the law of error propagation and statistical theory, and especially takes the high-repetition frequency characteristic of 3D GRICI into consideration. The model can theoretically guide the setting of the GRICI system parameters to obtain a higher range resolution compared with existing modeling methods. This paper also verifies the correctness of the proposed model through simulation and experiments, and quantitatively analyzes the influence of the accumulated pulse number in a single frame. In addition, the range resolution for our 3D GRICI system is measured under the guidance of the proposed model, and it reaches the millimeter order.