it was normal when i tried prevew. but just after bake, stain is everywhere. what happen?
my project link: https://landdecor.shapespark.com/example_thematrixone_landdecor/
It looks like there are some lights generating this noise after bake and the denoiser cannot remove it completely. Looks like in your case there are lights placed very close to objects and generating very small but strong reflections on these objects. Make sure the lights are not so close to geometry.
To detect such lights it’d be best to turn off all the lights, including emissive materials, except the natural and external ones. To turn off a light it’s enough to set its strength to 0. Then, you could enable some of them, and check if there is visible noise on the ceiling after the bake. I think you wouldn’t have to use high-quality baking settings for this investigation, 100-200 samples should be enough to differentiate between bakes with low and high amount of noise.
hello Magda,
thanks for reply
i have tried turn off all lights, then bake test. but stain is much more than. cant understand where i am wrong 
update 2: turn off sky mapping.
i uploaded my file to here. you can use it.
https://drive.google.com/file/d/13B3AEmIa8-rHjpF1leXoq6JEWYyRkWPd/view?usp=sharing
i cant find out how to solve this problem. do anyone have tips? 
Start from using sky & sun light sources, and using artificial light sources only to illuminate the hall: a few spot lights should be enough. You should get a decent bake using 200 samples and a clean one using 800. Then, step by step add additional light types, checking if they don’t introduce noise = stains.
Avoid light sources placed too close nearby objects or too strong light sources - they introduce significant noise and require much higher sample count during the baking, even up to 1600.