I want to set null to the value<=0 using the raster calculator in ArcMap, but after that, I got a new raster with a changed maximum value. Is there any problem with my setting? How to keep the other value (e.g. maximum value) the same but set null to value<=0? please see the attached screenshot.
You should run the Calculate Statistics tool on your original raster.
This is to ensure that the raster statistics (min, max, stdev etc) are correct, not approximate. You should never rely on the min/max values displayed in the layer legend in the table of contents, they can be approximate (derived from a sample).