A kind of optical fiber radiation pyrometer based on the theory of color comparison temperature measurement is designed in order to get the precise temperature on the appearance of work pieces during the process of plasma hot machining. In this article, the advantage of temperature measurement by optical fiber, the building of mathematic module for colorimetric temperature measurement, the application of key technology during the process of system building and the methods of reducing the interference is described in detail. The data calibrated and tested in blackbody furnace is given out. The results show that the precision of temperature measurement could reach to 0.5%. It is repetitiveness is well enough.