进入S3对应的存储桶，上面选项选权限，存储桶策略 -- 编辑，输入对应的policy。

完全控制，包含上传删除权限，policy如下：

{"Version": "2012-10-17","Statement": [{"Sid": "cross","Effect": "Allow","Principal": {"AWS": "arn:aws:iam::123456789:root"},"Action": "s3:*","Resource": ["arn:aws:s3:::bucket","arn:aws:s3:::bucket/*"]}]
}

只读权限policy：

{"Version": "2012-10-17","Statement": [{"Effect": "Allow","Principal": {"AWS": "arn:aws:iam::123456789:root"},"Action": "S3:ListBucket","Resource": "arn:aws:s3:::bucket","Condition": {}},{"Effect": "Allow","Principal": {"AWS": "arn:aws:iam::123456789:root"},"Action": "s3:GetObject","Resource": "arn:aws:s3:::bucket/*","Condition": {}}]
}

其中：

arn:aws:iam::123456789:root --> 123456789替换为对方aws 账户id

arn:aws:s3:::bucket --> bucket替换为共享的S3存储桶名

然后对方账户下即可通过IAM或者Role访问这个S3