What version of Lightroom? What export settings (especially under File Sizing)? What software are you using outside of Lightroom to view the results?
it's LR 5.6.
image sizing not checked only 240 pixel per inch.
I try everything: photoshop, macos preview etc.
This issue affects only images at 3200 iso or higher.
What camera model and how do the images look in the Library module. LR's Develop module only shows an accurate preview at 1:1 Zoom view, especially with high ISO images.The Library module will give you a more accurate indication of what Export images will look like when viewed at less than 1:1 Zoom view. I suspect the issue is that you need to apply a higher Luminance Noise Reduction setting, which you should do at 1:1 Zoom view in the Develop module.
As Todd suggests, compare the Develop and Exported images at 1:1 or 100% zoom, and see if they look different; otherwise you're at the mercy of whatever non-accurate resampling algorithm the viewer or LR is using to show non-1:1. You might also reimport the JPG into LR and compare it using the X|Y mode in Library at 1:1 just so we know that difference in color management aren't affecting things.
Do you have a sample DNG you can export of the raw file that you can share? Something that you don't mind sharing on a public forum? It can be a reject that is blurry or out-of-focus as long as it looks different after Export. Do an Export as a DNG with all your settings applied and upload to somewhere like www.dropbox.com and post a public download link, here.
Here I am:
You can find the Dng and an exported Jpg.
camera: Leica Monochrom, iso 3200
The original raw file is under-exposed by more than 4.0 EV.
ISO 3,200 - 4 EV = 51,200 Equivalent ISO
The raw file noise-level is extremely high and you have applied no Luminance noise reduction. This is causing the difference between the Develop module preview at less than 1:1 Zoom view when compared to Library or Export image. The raw image file is barely usable.
I suggest reviewing your exposure settings in the future. This may help: