In order to realize precise measurement for pF level capacitor, a capacitance measurement system based on laser locking Fabry-Perot interferometer was established and its basic principle, method of interferometer locking and determination of order numbers variation were studied. Firstly, according to the Lampard and Thompson electrical theorem, configurations and operation modes of Fabry-Perot interferometer were presented, and a displacement measurement by locking interferometer was demonstrated.Then the optical layout for Fabry-Perot interferometer was presented, and the operation status of the interferometer locking servo system and the fluctuation of cavity length in close loop were carefully analyzed. After the feasibility of determining the order numbers through the prior value of 1 pF standard capacitor was evaluated, the experiment process for confirming the variation of order numbers was presented. Finally, the character of step moving displacement and measurement repeatability of 1 pF standard capacitor were discussed. Experimental results indicate that the peak to peak fluctuation of the interferometer is 0.4 nm after its locking, and the repeatability of 1 pF standard capacitor measurement is 5.0×10-9. It can satisfy the requirements of precisely measuring the pF level capacitor. However, further study of the displacement measurement will focus on the uncertainty evaluation and optimization for the displacement measurement system.