最重要的是update的区域要指定整个panel，而不是想更新的那个组件

<h:form id="frm">

    <h:panelGrid id="panelgrid">

        <p:panel id="panel" >

        <h:outputLabel value="Name: " for="name" />

            <p:inputText id="name" value="#{userBean.name}" immediate="true" onkeypress="#{userBean.showName}">

                <p:ajax event="change" update="frm:panelgrid" listener="#{userBean.inputcangeeListener}" />

            </p:inputText>

        </p:panel>

            <p:panel id="p2" rendered="#{userBean.displayName}">

                <p:selectOneRadio id="city" value="#{userBean.city}" layout="grid" columns="3">

                    <f:selectItems value="#{userBean.cities}" var="c" itemLabel="#{city}" itemValue="#{city}" />

                </p:selectOneRadio>

            </p:panel>

        </h:panelGrid>

    </h:form>

And my managed bean looks like-

    @PostConstruct

    public void init() {

        displayName = false;

    }

    public boolean isShowName(){

        return displayName;

    }

    public void inputcangeeListener(javax.faces.event.AjaxBehaviorEvent changeEvent){

        setDisplayName(true);

        cities = new ArrayList<>();

        cities.add("pune");

        cities.add("KOL");

    }