As an important security component in the chip, true random number generator (TRNG) plays an increasingly important role in modern encryption systems. For the design of TRNG, the key is to require an entropy extractor to stably generate entropy under severe environmental changes (such as process fluctuations, voltage and temperature (PVT)). Based on the Xilinx FPGA platform, a lowcost, highefficiency true random number generator based on ring oscillator was proposed in this paper. The proposed TRNG improves the efficiency of entropy extraction through fast carry logic on the one hand, and optimizes the circuit structure and delay on the other hand to achieve considerable throughput and randomness with relatively low resource overhead. The TRNG proposed was verified on multiple Xilinx Virtex6 FPGAs and Xilinx Spartan6 FPGAs. Experimental data test results show that the proposed TRNG can exhibit good robustness in a wide range of PVT and generate random bit streams. Random bits only passed the NIST SP80022 statistical test suite with a fairly high P value, but also passed the latest NIST SP80090B test.