只需指定对象键Body可以保存的类型，如下所示：

{ Bucket: string; Key: string; Body: string | ReadStream }

解决方案：

import fs, { ReadStream } from "fs";
import path from "path";

const bucketName = '';
const uploadParams: { Bucket: string; Key: string; Body: string | ReadStream } = {
  Bucket: bucketName,
  Key: '',
  Body: '',
};
const file = '/home/a/bars/img1.png';

const fileStream = fs.createReadStream(file);
fileStream.on('error', function (err) {
  console.log('File Error', err);
});
uploadParams.Body = fileStream;
uploadParams.Key = path.basename(file);

或者，如果使用require，则可以使用typeof关键字：

// { Bucket: string; Key: string; Body: string | typeof ReadStream } 

const { ReadStream } = require("fs");
const fs = require("fs");
const path = require("path");

const bucketName = '';
const uploadParams: { Bucket: string; Key: string; Body: string | typeof ReadStream } = {
  Bucket: bucketName,
  Key: '',
  Body: '',
};
const file = '/home/a/bars/img1.png';

const fileStream = fs.createReadStream(file);
fileStream.on('error', function (err) {
  console.log('File Error', err);
});
uploadParams.Body = fileStream;
uploadParams.Key = path.basename(file);

虽然我觉得用空字符串来保存流很奇怪，但为什么不直接这么做呢？

const uploadParams: { Bucket: string; Key: string; Body: null | ReadStream } = {
  Bucket: bucketName,
  Key: '',
  Body: null,
};

示范-TSPlayground