The invention relates to the technical field of oil and gas logging while drilling, in particular to a signal excitation device of a horizontal principal ground stress direction measurement while drilling system. An excitation signal of the signal excitation device of the horizontal principal ground stress direction measurement while drilling system is a sine wave signal, the frequency can be adjusted to be the same as the working frequency of an ultrasonic transducer, it is guaranteed that signal energy can be transmitted to the well wall from the ultrasonic transducer to the maximum limit, the high signal output efficiency can be achieved, and meanwhile, a signal trailing phenomenon caused by mismatched frequency bands of a signal excitation unit and an ultrasonic transducer unit is avoided.