Game interventions have drawn much attention for their playful features and “painless” promotion of simple game-targeted pro-environmental behaviors (GPEBs), which could spill over to more “costly” prosocial behaviors for pro-environmental causes (PSBPs). The literature places much emphasis on game-targeted mechanisms (e.g., enjoyment and competition) to explain the impacts of game interventions, but pays little attention to non-game-targeted mechanisms to explore their spillover effect. Using online survey data from 1246 Chinese residents, this paper first compares players’ and nonplayers’ levels of engagement with GPEBs and with two common PSBPs, finding a positive spillover effect from game-targeted simple behaviors to more diverse PSBPs. Based upon goal theory, self-perception theory, and learning theory, the paper then examines the underlying mediating mechanisms of the positive relationship between GPEBs and PSBPs for subsamples of players and nonplayers respectively. The results demonstrate that individual awareness of problem and self-efficacy could be enhanced through engaging in simple GPEBs, which could spill over to more challenging and “costly” PSBPs, including charitable giving and volunteering for pro-environmental causes. The study provides important insights on helping to encourage specific types of more challenging PSBPs not directly targeted by the game.