The influence of loaded quality factor （QL） on the oscillator phase noise is analyzed on the basis of Leeson model in this paper, the expression of QL is obtained from the analysis of Colpitts oscillator circuit and the exact relationship between QL and the circuit parameters is clarified. The phase noise of the 50 MHz Colpitts crystal oscillator is simulated with Agilent ADS software. Based on the simulation results and improving the loaded quality factor QL, a prototype crystal oscillator is designed. The prototype uses the ATcut 3rd overtone crystal resonator with 49U resistance welding package, its unloaded quality factor （Q0） is about 1.45×105. The measured phase noise are better than -107 dBc/Hz@10 Hz, -134 dBc/Hz@100 Hz and -152 dBc/Hz@1 kHz. The experimental results show that it is feasible to design low phase noise crystal oscillators on the basis of improving QL.