It has been long time for lighting designers to solve the problem of lighting in some special rooms such as high temperature and humidity rooms. In this paper, a seamless prism light guide made by extruding technique and its application in a concrete-curing room with high temperature and humidity is introduced. The illuminance level and uniformity of the room has been improved greatly by application of the guide. The average illuminance on working surface created by the seamless light guide system is 19.2 lux while that created by the original lighting fixtures is 6.2 lux. The application of the seamless light guide system also makes the maintaining work easier and safer.