The deflection should be measured in fullscale structural testing of wind turbine blade. At present, a drawwire displacement sensor is used to measure the deflection. But the drawwire displacement sensor can only measure in a single direction, and the measurement error is large and the dimension is single. In order to solve these problems, a mathematical model for accurate measurement of threedimensional deflection of the blade was deduced. Furthermore, a wireless measurement system was built based on ultra wide band wireless ranging technology. The system was used for static load deflection measurement experiment of the blade, and the results were compared with those of laser tracker. The test results show that using the technology, accurate deflection values can be obtained, the root mean square errors of the measured blade deformation of X,Y,Z direction are 12.77, 15.91 and 19.87 mm respectively, which meet the test requirements.