it's known that salt concentration change is one of the factor of protein denaturation , but what is the basic behind it?
thank you

Hello,

A change in salt concentration means more or less positive and negative ions available for various interactions which are involved in keeping the protein together and helping it to hold it's structure.

A simple question with a complex answer! There are a few things going on. First, as Reetika said, charge balance is important for protein stability, because a protein uses charged residues (as well as other factors) to fold and stay folded. Heaps of ions in solution can mask charges and eliminate or severely curtail those interactions, potentially exposing internal hydrophobic regions and reducing protein solubility.
But perhaps more importantly, water-soluble proteins have concentric "shells" of semi-ordered water molecules arranged around them, in much the same way that dissolved salts have associated water molecules making them soluble. If you add too much salt, the waters in the protein solvation shells are stripped out to dissolve the salt, precipitating the protein out of solution. This is sometimes called "salting out".