The present invention discloses a method for transmitting the data which is modulated by multiple-input multiple-output space-time block coding, and a data transmission device which is applied in a data transmission system. The method comprises the following components: executing multiple-input multiple-output blocking code serial concatenation trellis coding to the information to be transmitted and regulating the position of the constellation point in the signal planisphere according to the length of the error event in which the least span of the maximization is 2; and mapping the coded signal on the signal planisphere for transmitting. The invention also discloses a device for transmitting the data which is modulated by multiple-input multiple-output space-time block coding. The data transmitting device according to the invention can obtain a high coding gain and at the same time saves the transmitting power of the transmitter.