The invention discloses a diagnostic method and system for overcharge of a battery of a hybrid electric vehicle. The method comprises the following steps: a sampling unit samples current, voltage, internal temperature, surface temperature and air temperature of a battery; an SOC calculation unit calculates the state of charge according to the current, the voltage and the internal temperature; a thermal model calculation unit calculates the heat production and the heat dissipating capacity according to the state of charge to obtain the predicted temperature and the predicted temperature rising rate; a temperature calculation unit calculates the difference between the internal temperature and the predicted temperature, the actual temperature rising rate and the difference between the actual temperature rising rate and the predicted temperature rising rate; a diagnosis unit judges the battery state according to the difference between the internal temperature and the predicted temperature and the difference between the actual temperature rising rate and the predicted temperature rising rate. The method and the system, which adopt the scheme, can judge whether the battery is overcharged or not timely and accurately, without being influenced by the environment temperature variation, and are suitable for all hybrid electric vehicles.