how to add querystring values with RedirectToAction method?

Any values that are passed that aren't part of the route will be used as querystring parameters:

return this.RedirectToAction
  ("myActionName", new { value1 = "queryStringValue1" });

Would return:

/controller/myActionName?value1=queryStringValue1

Assuming there's no route parameter named "value1".

For people like me who were looking to add the CURRENT querystring values to the RedirectToAction, this is the solution:

var routeValuesDictionary = new RouteValueDictionary();
Request.QueryString.AllKeys.ForEach(key => routeValuesDictionary.Add(key, Request.QueryString[key]));
routeValuesDictionary.Add("AnotherFixedParm", "true");
RedirectToAction("ActionName", "Controller", routeValuesDictionary);

The solution as you can see is to use the RouteValueDictionary object

If you already have a query string from somewhere, you can skip parsing and reassembling the query string and just use Redirect with Url.Action:

string queryString ="?param1=xxx&page=5";
return Redirect(Url.Action("MyAction") + queryString);