ASP.NET Core 3 使用 System.Text.Json 而不是 Newtonsoft.Json(又名 JSON).NET) 来处理 JSON.JSON.NET 支持从字符串解析为十进制，但 System.Text.Json 不支持.如文档:

ASP.NET Core 3 uses System.Text.Json instead of Newtonsoft.Json (aka JSON.NET) for handling JSON. JSON.NET supports parsing from a string into a decimal, but System.Text.Json does not. The easiest thing to do at this stage is to switch back to using JSON.NET, as described in the docs:

• 添加对 Microsoft.AspNetCore.Mvc.NewtonsoftJson 的包引用.

• 更新 Startup.ConfigureServices 以调用 AddNewtonsoftJson.

• Add a package reference to Microsoft.AspNetCore.Mvc.NewtonsoftJson.

• Update Startup.ConfigureServices to call AddNewtonsoftJson.

services.AddMvc()
      .AddNewtonsoftJson();

将十进制数作为 JSON 数字传递会更正确，但很明显这不适合您.

It would be more correct to pass the decimal as a JSON number, but it's clear that's not going to be an option for you.