The disclosure discloses a cooling system and an automatic coolant injection method for the cooling system. The cooling system includes a heat exchanger; a converter; a liquid cooling pipeline; a coolant tank with a liquid-level sensor; an injection pump for injecting coolant from the coolant tank into the liquid cooling pipe; and a control unit. When liquid level of the coolant tank reaches an upper threshold, the injection pump injects coolant into the liquid cooling pipeline. When pressure of the coolant in the liquid cooling pipe reaches an upper static liquid pressure threshold, the control unit turns off the injection pump, and executes the turning-on and turning-off operations of the circulation pump with a preset circulation period. The circulation pump forces the coolant to circulate in the liquid cooling pipeline when being turned on.