Particle drag is a very important factor in reactor simulation. The complex physical properties of supercritical water (SCW) prevent some modeling methods of reactor simulation from being able to accurately simulate an SCW reactor. Therefore, in this work, the effects of particle interaction on single particle drag within a multi-particle system in SCW are investigated. The results show that the variation in the drag coefficient in SCW is special. This work indicates a control mechanism for the Reynolds number, volume fraction, temperature, and pressure on drag specificity of a single particle. This mechanism essentially exhibits an interaction of viscosity and velocity gradient. Furthermore, through a comparison of SCW and constant property flow, a drag specificity model can be initially developed. The results for SCW can be obtained by calculating the constant property flow, coupled with a drag specificity model. This model can be applied to modeling methods of reactor simulation after further improvement.