An auto testing,estimating and calibrating system for online monitoring system of gases dissolved in transformer oil is designed and researched. The system can prepared standard oils with low, middle or high dissolved-gases concentrations in the closed tank automatically，based on a thermodynamic model of gas_liquid equilibrium. The automatic collection and analysis of verification data are optimized, and automatic calibration of gas concentration and intelligent analysis are realized. It effectively overcomes the shortcomings of large error and low efficiency of manual verification. In addition, the data curve fitting based on the least square method reduces the detection error of the on-line monitoring device.