Abstract: An entropy maintaining method for a true random number generator and a true random number generator thereof are provided. The true random number generator includes a plurality of first ring oscillation output circuits. The entropy maintaining method includes providing at least one second ring oscillation output circuit in the true random number generator, the at least one second ring oscillation output circuit initially disabled; after the true random number generator is operated, recording logic of generated random bit signals every preset time; counting the logic of the random number bit signal to obtain a statistic value; and determining, according to the statistic value, whether the at least one second ring oscillation output circuit is operated in combination with the plurality of first ring oscillation output circuits to generate the random bit signals.