The invention discloses an approximation online flutter boundary prediction method. The method comprises a step of performing a blowing test at a continuous wind speed in a low-speed wind tunnel and measuring a vibration response of a key part of an aircraft, a step of drawing each mode frequency peak reciprocal map, wherein an obvious downward trend means a dangerous mode, a step of continuing tocarry out an air blow test at a continuous wind speed, collecting data, only reading a maximum value near a frequency of the dangerous mode to be recorded as a frequency peak, wherein each data pointhas a one-to-one corresponding measurement wind speed, and an average value of speeds corresponding to the data of the point is a speed matched with the peak, a step of drawing a relational graph between a peak reciprocal and a speed and carrying out fitting and extrapolation by using a linear function to obtain a flutter boundary speed, and a step of continuing to collect and process new test data, continuously obtaining a new flutter boundary speed, and stopping the test if a predicted relative error between the flutter boundary speed and an actual blowing speed is smaller than 3%. According to the method, the flutter boundary prediction at the continuous wind speed is realized, and the method is an approximate online prediction method.