AWS S3预签名URL与自定义策略结合使用可以实现更精细的访问控制。下面是一个使用Python语言编写的示例代码，演示了如何生成具有自定义策略的S3预签名URL：

import boto3
from botocore.exceptions import ClientError
import datetime

def generate_presigned_url(bucket_name, object_name, expiration=3600, custom_policy=None):
    """Generate a presigned URL to share an S3 object

    :param bucket_name: string
    :param object_name: string
    :param expiration: Time in seconds for the presigned URL to remain valid
    :param custom_policy: Custom policy string as JSON format
    :return: Presigned URL as string. If error, returns None.
    """

    # Generate a presigned URL for the S3 object
    s3_client = boto3.client('s3')
    try:
        params = {
            'Bucket': bucket_name,
            'Key': object_name
        }
        if custom_policy:
            params['Policy'] = custom_policy
        
        url = s3_client.generate_presigned_url(
            'get_object',
            Params=params,
            ExpiresIn=expiration
        )

        return url
    except ClientError as e:
        print(e)
        return None

# 示例用法
bucket = 'your-bucket-name'
object_key = 'your-object-key'

# 创建自定义策略
policy = {
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "AllowGetObject",
            "Effect": "Allow",
            "Principal": "*",
            "Action": "s3:GetObject",
            "Resource": f"arn:aws:s3:::{bucket}/{object_key}",
            "Condition": {
                "IpAddress": {
                    "aws:SourceIp": "192.168.0.1/24"
                }
            }
        }
    ]
}

# 将策略转换为JSON字符串
import json
policy_json = json.dumps(policy)

# 生成预签名URL
url = generate_presigned_url(bucket, object_key, expiration=3600, custom_policy=policy_json)
print(url)

上述代码中的generate_presigned_url函数用于生成带有自定义策略的S3预签名URL。你可以根据自己的需求修改策略中的条件和资源等信息。

注意，预签名URL的有效期可以自行设置，默认为3600秒。此外，代码中使用的策略为示例策略，你需要根据自己的需求自行编写适用的策略。

希望对你有所帮助！