When applied in a high temperature environment simulation system, computer data acquisition fluctuates dramatically as the instability of the nonstandard thermocouple transmitter. A method which sending the thermocouple voltage to the computer directly is designed. Then a fitting function is acquired by curve fitting in poly-least-square, which could be used to avoid the waste of computer resource as the comparing table-searching work in program. And it could be seen that the fitting error is around ±3℃ in the high temperature area through error analysis. The fitting formula could be used in the signal acquisition of this kind of thermocouple. And the fitting degree could be adjusted according to the measurement precision.