The invention relates to a signal transmitting device for realizing wireless measurement while drilling. A downhole signal transmitting device for an electromagnetic measurement while drilling system comprises an upper drilling string and a lower drilling string, and an insulating joint is contained between the upper drilling string and the lower drilling string; and the upper drilling string andthe lower drilling string are used for forming an asymmetric dipole antenna. The transmitting device adopts software radio technology to realize various functions of wireless transmission; the impedance matching between a transmitter antenna and transmission medium is realized through an impedance converter; and the dipole antenna is adopted to transmit a measurement while drilling data signal. The device and the method improve the adaptive capacity and measurement depth of the electromagnetic measurement while drilling system to different strata.