A kind of measuring instrument based on F-P interference wave length is designed.The measuring instrument use double interference cavities.One of interference cavities is reference cavity,the other is measuring cavity.One reflect surface of reference cavity is installed with piezoelectric ceramics,One reflect surface of measuring cavity is erected with an object to be measured.When the central wavelengths in reference Fabry-Perot interference and in measuring Fabry-Perot interference are overlapped,there are the maximum light signal outputs. Based on the relationship between the transmission spectrum central wave length and its interference cavity length, this measuring instrument can be used to realize the real time measurement of Nano-grade micro-displacement.Using simi-conductor laser as the light resource can obtain the required wavelength values and its variation scopes.It can be seen from the measured results that the measured error is not over 1.5 nm. Such a precision can satisfy the precision requirements as in the ......