The invention provides a belt tension force detector of a gasoline engine and a diesel engine and a detecting method thereof, wherein the belt tension force detector comprises a mechanical pressure gauge, a deflection reading block and a straight panel; the mechanical pressure gauge is installed above the deflection reading block, and the straight panel is arranged below the deflection reading block. By the belt tension force detector, certain pressure is applied to the middle part of the belt between two wheels, the span of which is maximum, in a wheel train, then the deflection of the belt is measured to judge whether the deflection of the belt is within the control range so as to detect whether the tension degree of the belt meets requirements or not. By only a mechanical pressure gauge which is common on the market, a unidirectional mechanism and a simple straight panel, the tension force of the belt can be measured; aiming at measurements of different spans, only a straight panelwith corresponding length needs to be made, thus greatly enlarging the use range; and simultaneously, the cost is very low. Therefore, the belt tension force detector is suitable for being used in general assembly shops and used by maintenance and detecting personnel in wide range.