string value = "one\r\ntwo\r\nthree";

label.Text = value.Replace(Environment.NewLine,"<br/>");

但是如果你有一个字符串列表,你可以尝试转发器的方法

<asp:Label ID="label" runat="server">
    <asp:Repeater ID="repeater" runat="server">
        <ItemTemplate>
            <%# Container.DataItem %> <br />
        </ItemTemplate>
    </asp:Repeater>
</asp:Label>

和代码

List<string> listOfStrings = new List<string>()
    {
        "One","Two","Three"
    };

repeater.DataSource = listOfStrings;
repeater.DataBind();