A system, client and method for transmitting preview message in an instant messaging system are disclosed by the present invention. And the system includes: a receiver (400) including a first preview message processing module (401), a first preview message display module (402) and a first receiving module (403); a sender (100) including a second preview message processing module (101) and a second sending module (104); an instant messaging server (200) for transmitting an instant customized message between the sender (100) and the receiver (400); and an Internet (300) for transmitting an instant message between the sender (100) and the receiver (400); wherein the second preview message processing module (101) is configured to convert the preview message received into a code, and transmit the code to the second sending module (104); the second sending module (104) is configured to send the code to the first receiving module (403) via the Internet (300); the first receiving module (403) is configured to send the code to the first preview message processing module (401); the first preview message processing module (401) is configured to convert the code into a prompt message indicating the preview message, and send the prompt message to the first preview message display module (402) for prompt.