The invention provides an open cooling water system of a gas thermal power plant and a start-stop controlling method thereof. The start-stop controlling method comprises the following steps: an initial starting step, a unit selecting step, an operation starting step and a system stopping step, wherein the method comprises a step of operating any water circulating pump as required and determining whether the pressure of a circulating water outlet mother pipe is smaller than a preset value and whether an open cooling water pump is selected in advance before the initial starting step; and the method also comprises a step of determining whether the open cooling water pump stops and a water ring vacuum pump stops as required before the system stopping step. Through the start-stop controlling method, the open cooling water system can be automatically started in a fully-stopped state or a partially-started state of the open cooling water system; the original operation state during partially starting is not disturbed; the open cooling water system can be automatically stopped in the operation state.