True random-number generators based on ring oscillators (RO-based TRNG) are widely used in the field of information encryption because of their simple structure and compatibility with CMOS technology. However, radiated or conducted electromagnetic interference can dramatically deteriorate the randomness of the output bitstream of the RO-based TRNG, which poses a great threat to security. Traditional research focuses on the innovation of the means of attack and the detection of circuit states. There is a lack of research on the interference mechanism and anti-interference countermeasures. In this paper, the response of the RO array to electromagnetic interference was analyzed, and the concept of synchronous locking was proposed to describe the locking scene of multiple ROs. On the basis of synchronous locking, the RF immunity of the RO-based TRNG was modeled, which can explain the degradation mechanism of bitstream randomness under RFI. Moreover, the design method of gate-delay differentiation is presented to improve the RF immunity of the RO-based TRNG at a low cost. Both transistor-level simulation and board-level measurement proved the rationality of this scheme.