The present invention provides a method, an apparatus, and a system for securely transmitting data. A method for securely transmitting data is provided, where the method includes: sending, by a user terminal, a resource access request carrying a first authentication header field to a server, where the first authentication header field includes a user identifier and a server identifier; and receiving a request response returned by the server, where the request response includes a second authentication header field and a message body, where the second authentication header field carries a third integrity digest, and the third integrity digest is obtained by the server by performing, after receiving the resource access request, calculation by using a third message-digest algorithm further according to a user password and message content; and the message body carries message content that is encrypted by using a shared key; where the user password is obtained from a storage area of the server according to the user identifier carried by the resource access request, so that M2M transmission based on the CoAP protocol can be performed securely and reliably.