The energy relaxation of particles in a background is a fundamental issue in nonequilibrium statistical dynamics. Single collisions between a nonequilibrium particle and background molecules serve as the elementary unit of the particle’s nonequilibrium stochastic evolution and relaxation processes. Studies of the statistical properties and distribution of energy exchange in single collisions therefore provide a solid foundation for exploring the long-time evolutionary behavior of the entire system or its relevant subsystems. In previous ours microscopic studies, it derived the mean energy gain equation for a particle with a specified incident speed undergoing a probabilistic collision with background molecules, yet its validity remains to be fully verified. Based on microscopic kinetic theory, a physical model for probabilistic collisions between an incident particle and background molecules is constructed, and corresponding numerical simulations are carried out. The balance speeds for particles of different masses-speeds at which the theoretical mean energy gain vanishes-are calculated numerically. Simulations are performed for a large number of collision events, and the statistical mean energy gains or losses are shown to be in excellent agreement with theoretical predictions, thus rigorously verifying the derived mean energy gain equation. For instance, at the balance speed, when the mass of the incident particle equals that of the background molecules, the energy gain is only on the order of 10^-5kBT, demonstrating extremely high consistency between simulation and theory. For off-balance speeds, the mean energy gain is found to be approximately proportional to the difference between the incident kinetic energy and the balance energy. The post-collision speed distribution is systematically analyzed over various mass ratios and incident speeds. The distribution exhibits a single peak and deviates significantly from the Maxwell-Boltzmann distribution, confirming that energy relaxation is a gradual process involving multiple successive collisions. For balance speed incidence, the peak is located very close to the incident speed, and its half-width decreases with increasing mass ratio. For off-balance speeds, the peak shifts away from the incident speed, especially at low and high incident speeds. Last but not least, at incident speeds far below the balance speed, the distribution becomes nearly independent of the incident speed, with the peak energy stabilizing around kBT/2. This behavior reflects the dominance of thermal fluctuations and may find useful applications in thermalization processes. This study provides a clear microscopic picture of the energy relaxation process of nonequilibrium particles in an equilibrium environment, offering physical insights into stochastic dynamics in the kinetic limit and reliable theoretical support for practical applications involving thermalization processes.