The embodiment of the invention provides a header height monitoring method for an agricultural machine and a header height sensor for the agricultural machine. The header height monitoring method comprises the steps of acquiring a real-time oblique angle of a bridge structure when a header is at a to-be-measured position; and determining a real-time height when the header is at the to-be-measuredposition according to a quantitative relation between the real-time oblique angle and the real-time height of the header. According to the header height monitoring method provided by the embodiment ofthe invention, the quantitative relation is determined according to basic data when the header is arranged at an initial position, the real-time height is further obtained, the obtained header heightcannot be affected by an operation environment, and accurate monitoring of the header height is achieved; and moreover, the real-time height of the header at an arbitrary to-be-measured position canbe determined once the basic data of the initial position is determined, and the defect that the header height cannot be determined after a stay wire sensor or a push rod sensor in the prior art is damaged is overcome.