Thank's for your answer
I fixed this problem with a better solution, with an extra option parameter for datagrid, because my field value there is in another database field.
This solution automatically work in "onBeginEdit" event of datagrid;
Example:
var editors = $(this).edatagrid('getEditors', index);
$.each(editors, function() {
if( this.type == 'combobox' ){
var opts = $(this.target).combobox('options');
$(this.target).combobox('setValue', row[ ( opts.defaultValueField != '' ? opts.defaultValueField : this.field ) ]);
}
});