Most therapeutic proteins except antibodies necessitate frequent dosing at high concentrations due to their short circulation half-lives, leading to limited therapeutic efficacy, serious adverse side effects and poor patient compliance. Herein we report a strategy of thermoresponsive polypeptide fusion to genetically engineer a super-long-acting interferon alpha fused with a body-temperature-responsive polypeptide. After a single subcutaneous injection in a mouse model, this interferon alpha can in situ form a depot to show a one-month zero-order sustained release, which would enable a once-trimonthly dosing in humans. As a result, it exhibits greatly enhanced tumor accumulation and tumor eradication as well as substantially improved tolerability and biosafety. This strategy provides a promising solution to dramatically enhance the pharmacological performance of therapeutic proteins with short circulation half-lives while reducing the side effects.