General method and theoretical basis for testing internal condition of heat equipment by infrared quantitative diagnosis are introduced. The testing technology of infrared is combined with the inverse heat conduction problem of determining inner wall temperature distribution of heat equipment. The common step to get the space step iteration formula of calculating inner temperature is also described by space separating and using finite difference methodes. The temperature of each point in the equipment is calculated numerically. So the internal condition of heat equipment can be test. The working process of this method is also stated by giving example of stable heat process in plane slab.