A method and a system for diagnosing insufficiency of vehicle coolant includes a preparing step of checking whether the vehicle is in a stop state and is on flat ground. An operating step applies a predefined output to a coolant water pump when the vehicle is in the stop state and is on flat ground. A measuring step measures a rotation speed of an impeller of the coolant water pump for a period in which the predefined output is applied to the coolant water pump. A M determining step compares the measured rotation speed of the impeller of the coolant water pump with a reference speed to determine whether or not the coolant is insufficient.