Two-dimensional (2D) ferromagnets are popular in fields such as spintronic devices, but their low Curie temperature ( T C ) limits their practical application. In this work, by using a global optimization evolutionary algorithm and density functional theory method, a Janus CrSSe ferromagnetic monolayer was predicted systematically. Monte Carlo simulations show that the Curie temperature of the Janus CrSSe monolayer is about 272 K and can be adjusted to 496 K under a small tensile biaxial strain. Besides, this monolayer possesses large magnetic anisotropy energy (1.4 meV Cr -1 ). The magnetic order can be changed from ferromagnetic to antiferromagnetic order under compressive strain. What's more, this monolayer possesses the lowest energy in the 2D search space and excellent thermal, dynamic, and mechanical stabilities. Considering its excellent properties and current experimental techniques, it is possible to synthesize CrSSe monolayer experimentally.