In the era of information explosion, the issues of information security are of broad concern. The physical unclonable function (PUF) and the true random number generator become the basic security primitives, providing a lightweight solution. An entropy source separation model is proposed in this paper, which can separate the delays caused by jitter (the entropy of the true random number generator) and process deviation (PUF entropy) in a ring oscillator. A reconfigurable dual-mode circuit on FPGA was designed, which could reconfigure the PUF circuit into a true random number generator circuit by changing the mode. The proposed structure has the advantages of small resource overhead, large area utilization and low power consumption. The experimental results show that the PUF generated by the proposed structure has excellent performance in stability, uniqueness, and uniformity. The true random numbers generated by the proposed structure have high randomness and unpredictability. In addition, all true random numbers have passed the NIST test.