The invention relates to a method for monitoring a rotating angle of a deep marine riser. The method comprises the following steps: (1) setting a marine riser tilt angle monitoring system comprising a rotating angle detection system, an underwater acoustic signal receiving device and a data monitoring center; (2) mounting a double-axis tilt angle sensor at a preset position of the marine riser before the marine riser enters water, controlling the double-axis tilt angle sensor to start working by virtue of a controller after the marine riser reaches the subsea wellhead, measuring the included angles alpha and beta between the marine riser and the axis X and between the marine riser and the axis Y by virtue of the double-axis tilt angle sensor, transmitting the measured tilt angle signals alpha and beta to a DSP, calculating to acquire a rotating angle signal gamma of the marine riser by virtue of the DSP according to a preset formula, and processing the rotating angle of the marine riser so as to spread by virtue of an underwater acoustic channel; (3) lifting the underwater acoustic signal receiving device at a certain position below the sea surface to be used for receiving the rotating angle signal of the marine riser, and transmitting the signal to the data monitoring center; and (4) displaying and saving the rotating angle signal data of the marine riser received by the data monitoring center, so that the platform staff can determine the operating state according to the rotating angle information of the marine riser displayed by the data monitoring center.