Abstract: In the progress of large aspheric mirror manufacturing, computer controlled active lap(CCAL) has been proved to be a useful tool with high precision and efficiency. Before polishing, the deformation measurement of the lap is necessary. The measurement system has 60 high precision displacement sensors. Error analysis and essential data processing of the measurement system are putting forward. The main error contains ball-probe error as the system error, angle error of fitting sensors and error of coordinate offset as the stochastic error. In the mathematic model established based on the characteristic of paraboloid and coordinate transform, the influence of the three kinds of errors is calculated. After some interrelated compensation and data processing, the result gotten shows that the measurement precision improves.