I want to use SharePoint REST API to get and operate the permission value of a specific folder in Shared Documents.

Hi @大坂 翔 ,
First we need to break role inheritance for the folder.

function breakRoleInheritanceOfFile() {   
	 $.ajax({  
			url: siteurl + String.format("/_api/web/GetFolderByServerRelativeUrl('DocumentTest/test999')/ListItemAllFields/breakroleinheritance(true)"),  
			type: "POST",  
			contentType: "application/json;odata=verbose",  
			headers: {  
				"Accept": "application/json;odata=verbose",  
				"X-RequestDigest": $("#__REQUESTDIGEST").val()  
			},  
			success: onQuerySucceeded,  
			error: onQueryFailed  
	 });     
}  

Remove the current role assignment for the folder

function remove(){  
	$.ajax({  
			url: siteurl + String.format("/_api/web/GetFolderByServerRelativeUrl('DocumentTest/test999')/ListItemAllFields/roleassignments/getbyprincipalid(11)"),  
			method: "POST",  
			headers: {  
				   Authorization: "Bearer " + accessToken,  
				   "accept": "application/json;odata=verbose",  
				   "content-type": "application/json;odata=verbose",  
				   "X-HTTP-Method": "DELETE"  
			},  
			success: onQuerySucceeded,  
			error: onQueryFailed  
	});   
}  

Add the new role assignment for the folder

function add(){  
	$.ajax({  
			url: siteurl + String.format("/_api/web/GetFolderByServerRelativeUrl('DocumentTest/test999')/ListItemAllFields/roleassignments/addroleassignment(principalid=11,roledefid=1073741829)"),  
			method: "POST",  
			headers: {  
				   Authorization: "Bearer " + accessToken,  
				   "accept": "application/json;odata=verbose",  
				   "content-type": "application/json;odata=verbose"  
			},  
			success: onQuerySucceeded,  
			error: onQueryFailed  
	}); 	  
}  

In order to use this API we need 'FullControl' permissions,So we need get Access Token by app

If an Answer is helpful, please click "Accept Answer" and upvote it.
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.