I've found the problem, the problem is that when the data loadcompleted, i set the datatype:local and sort for a column, then ts.p.lastsort remembered this column.

After i set the datatype:json and reload the grid the ts.p.lastsort is also exist,so it will not re-sort this out but the implementation of the reverse.

So i add code in grid.base.js at line 1514 "ts.p.lastsort = null;", problem solved.

bind('reloadGrid', function(e, opts) {
                if (ts.p.treeGrid === true) { ts.p.datatype = ts.p.treedatatype; }
                if (opts && opts.current) {
                    ts.grid.selectionPreserver(ts);
                }
                if (ts.p.datatype == "local") { $(ts).jqGrid("resetSelection"); }
                else if (!ts.p.treeGrid) {
                    ts.p.selrow = null;
                    ts.p.lastsort = null;
                    if (ts.p.multiselect) { ts.p.selarrrow = []; $('#cb_' + $.jgrid.jqID(ts.p.id), ts.grid.hDiv).attr("checked", false); }
                    ts.p.savedRow = [];
                    if (ts.p.scroll) { emptyRows(ts.grid.bDiv, true); }
                }