The invention discloses a highway traffic flow space average speed observation method based on a fixed wing model airplane. The method is applied to the fixed wing model airplane with specific functions flying in a fixed height, evenly and in an inverse traffic stream direction, photographic equipment is carried to take traffic stream photos at specific time intervals, photos of the measured and calculated space are extracted, vehicle changing numbers in two adjacent photos are counted, data of relative vehicle streams, traffic stream density, time intervals, relative speeds and the like are obtained, and then space average speed is calculated and obtained. The highway traffic stream space average speed observation method based on the fixed wing model airplane is simple, fast and convenient, observation results are reasonable and reliable, and the problem that an existing method cannot directly measure and calculate the space average speed is solved.