The dependence of the quantum correlation of twin beams generated from a non-degenerate optical parametric oscillator on the analysis frequency was theoretically studied with semi-classical method. It was pointed out that the quantum entanglement is higher at lower analysis frequency. By means of a pair of unbalanced fiber Mach-Zehnder interferometers，the correlation noise spectrum of the intensity-difference and the phase-sum of twin beams with non-degenerate frequencies and total power of 20mW were experimentally measured，which were 3.1 and 1.3dB below the corresponding shot noise limit at 2MHz，respectively. The experimental results are in agreement with the theoretical predictions.