Due to the impact of temperature, voltage and device aging, the traditional Ring Oscillator based Physical Unclonable Functions (RO-PUF) suffer from two main issues, unreliability of Physical Unclonable Functions (PUF) response and nonrandom distribution of Ring Oscillator (RO) frequencies. In order to improve the PUF reliability, an approximate frequency slope compensation method that uses the slope relationship between RO frequency and temperature to compensate the instable RO frequency was proposed in this paper. As a result, the instable Challenge-Response Pairs (CRP) perform the reliable responses. To enhance the security, a new scheme based on Mean Absolute Difference (MAD) was proposed in this paper. Firstly, the scheme measured the RO's average frequency of each chip, then filtered the corresponding RO's average frequency multiple times to extract the true random frequency. Consequently, the output of PUF follows the random distribution. The experimental results show that the proposed scheme can improve RO-PUF's reliability and security effectively.