An optical MEMS pressure sensor based on a mesa-diaphragm is presented. The operating principle of the sensor is expatiated by Fabry-Perot (F-P) interference. Both the mechanical model and the signal averaging effect of the mesa diaphragm is validated by simulation, which declares that the mesa diaphragm is superior to the planar one on the parallelism and can reduce the signal averaging effect. Experimental results demonstrate that the mesa structure sensor has a reasonable linearity and sensitivity.