Abstract: One of the key factors ensuring safe drilling of dense cluster wells is the overall anti-collision design of borehole trajectories. To reduce borehole collision risks, parameters for the design of borehole trajectories in upper sections must be properly selected and optimized. China’s standards in prior art were compared to analyze how they guide and apply to the overall anti-collision design of borehole trajectories for dense cluster wells. A separation coefficient in the analysis on collision risks of actual borehole trajectories was introduced. Critical values of four key parameters (KOP depth, offset well KOP depth difference, build-up rate and angle-building azimuth) for upper sections of dense cluster wells were calculated and analyzed with the separation coefficient being 1. Study results show the following: a smaller wellhead spacing delivers a smaller critical KOP depth, a smaller KOP depth difference, a smaller angle-building azimuth and a bigger critical build-up rate; the offset well KOP depth difference defined in existing standards has sufficient safety margins and can be properly decreased for the anti-collision design of infill adjustment wells.