An improved heat collector in a fluid-cooling system for a computer includes a case, and a cooling plate. The case comprises a water inlet, a water outlet, a top surface, and a bottom surface, wherein the bottom surface is a heat collecting face. The cooling plate is firmly connected to the top surface of the case, the cooling plate further comprising a plurality of cooling fins protruded upwards. A natural airflow can be formed among the cooling fins of the heat collector; therefore the heat collector can also radiate heat to air to improve the cooling efficiency.