使用AWS SDK提供的s3.getObject方法，将S3对象的内容读取到内存中，然后将其写入Lambda函数的/tmp目录下的临时文件。

以下是一个Node.js示例示范了如何处理此问题：

const AWS = require('aws-sdk');
const fs = require('fs');

exports.handler = async (event, context) => {
  const s3 = new AWS.S3();
  const params = {
    Bucket: 'my-bucket',
    Key: 'my-file.txt'
  };
  
  const fileContent = await s3.getObject(params).promise();
  
  // Write the object content to a temporary file
  fs.writeFileSync('/tmp/my-file.txt', fileContent.Body);

  // Process the file
  // ...

  return {
    statusCode: 200,
    body: 'File processed successfully'
  };
};