This invention provides a method and system for data encryption and decryption in data transmission through the web. The method includes: a browser sends a cryptographic information acquisition request to a cryptographic information providing equipment; the cryptographic information providing equipment sends cryptographic information back to the browser via an HTTPS channel; the cryptographic information includes a cryptographic algorithm and a cryptographic index; the browser uses the cryptographic algorithm to encrypt the data to be transmitted, and sends the encrypted data and the cryptographic index to the web server via an HTTP channel; the web server obtains the cryptographic algorithm corresponding to the cryptographic index from the cryptographic information providing equipment, then decrypts the encrypted data. Embodiments of the present invention can alleviate the load in the HTTPS channel, and improve the overall performance.