I guess you have a query string in the action attribute of the form which is getting list when the form is submitted. This has nothing to do with struts. Its the way browsers work. Try either POST method for the form or create hidden form fields in your form instead of query string...
when the method execute is finished i go to another page or to the same page (it depends from result name attribute) but i loose the param i have in the url before clicking the submit of the form; how can avoid this behaviour?
for example i remember that in struts 1 i could decide the return page from the code of my action class e i could add some param to this url