I found an error when posting features to postgis by openlayers and geoserver. The code is listed below. And the error was that the var 'data' which was converted from the var 'node' by XMLSerializer was wrong, the order of X and Y in it was reverse, just like "28.795251846313476 116.4409589767456 ". So geoserver rejected the request, because the coordinates of Y were outside of (-90,90).
var feature = evt.feature;
var node = format.writeTransaction([feature], null, null, {
gmlOptions: {srsName: "EPSG:4326"},
featureNS: "ucoc.zhtx.com",
featureType: "landblock"
});
var data=new XMLSerializer().serializeToString(node);
Welcome to the wonderful world of axis orientation trouble. Fortunately, GeoServer has an alternative SRS code for [lon, lat] order, which is "CRS:84" instead of "EPSG:4326":