looked at the source code of Microsoft.Extensions.Configuration .
found that JsonConfigurationProvider 's base class ConfigurationProvider has a protected void OnReload () , so the solution is clear.
inherits JsonConfigurationProvider to create a new Provider , and then override OnReload method.

then in Program.cs

public static IWebHostBuilder CreateWebHostBuilder(string[] args) =>
    WebHost.CreateDefaultBuilder(args)
        .ConfigureAppConfiguration((builderContext, config) =>
         {     
            IHostingEnvironment env = builderContext.HostingEnvironment;
             var c = config as Microsoft.Extensions.Configuration.ConfigurationBuilder;
            //c.Sourcesappsettings.json  JsonConfigurationProvider
            //c.SourcesProviderc.SourcesList
         })
         .UseStartup<Startup>();