The PC light diffuser has an unchanging frame with good sealing performance and placement techniques. The upper end of the unchanging shell rolls and continues to be assembled in a limited position, and the inside of the unchanging shell continues to have a guard shell, and the appearance of the guard shell is fixed. The inner wall of the guard shell remains unchanged and there is a guide body, the inner wall of the unchanging shell and the guard shell are continuously pasted with a sealing ring, and the corners of the unchanging shell are provided with unchanging grooves. The outer side of the lower end of the shell is attached to the upper end surface of the invariant block, and the single body of the adjusting rod undergoes a telescopic rod to continue. It has a constant frame with good sealing function, which facilitates the adjustment of the size and size of the guide in accordance with the difference of size and size, and also facilitates the sealing of the PC light diffusion plate, avoiding the leakage of the LED light, facilitates the installation and damage, and is convenient for replacement .