An auto testing, estimating and calibrating system for online monitoring system of gases dissolved in transformer oil is designed and researched in this paper. The system adopts the oil circulation gas-liquid equilibrium mode to prepare standard oils with low, middle and high dissolved-gases concentrations in the closed tank automatically, and the automatic oil sample testing, data collection, results calculation and estimation are realized. It effectively overcomes the shortcomings of large error and low efficiency of manual verification. In addition, the data curve fitting is conducted based on the least square method,which reduces the detection error of the on-line monitoring device.