The invention is applicable to the technical field of reactor control and protection systems, and provides a parameter calibration method, a device and a system of a nuclear instrument system. The method comprises steps of: obtaining current data of a ionization chamber in the nuclear instrument system; calculating an upper part current and a part lower current according to the current data, wherein the upper part current is a current in an upper half part of the ionization chamber, the lower current is a current in a lower half part of the ionization chamber; according to the upper part current and the lower part current, obtaining a second nuclear instrument system parameter through inverse calculation of a nuclear power calculation formula of the nuclear instrument system, wherein the second nuclear instrument system parameter is a new nuclear instrument system parameter. According to the parameter calibration method, the device and the system of the nuclear instrument system, the parameter giving time is saved, manual reading and data inputting are not needed, the working efficiency is improved, the error risk is reduced, and the unit output capacity is improved.