Using the active flow control method to accelerate the attenuation and fragmentation of the wing tip vortex is one of the most potential technologies to improve the take-off and landing frequency of the airport and ensure the flight safety of aircraft. Due to the lack of understanding of wingtip vortex instability， the existing active control methods often cannot achieve the optimal control effect. To reveal the evolution law of the unstable modes of the wingtip vortex， the evolution characteristics of the unstable modes of the isolated wingtip vortex are studied using the SPIV technique and the linear stability analysis method. The results show that： the perturbation modes of the isolated wingtip vortex can be divided into four types according to its position in the eigenvalue spectrum： main perturbation mode， P secondary perturbation mode， A secondary perturbation mode and S secondary perturbation mode. Among them， the main perturbation mode and P-group perturbation mode have two-lobe structural characteristics， which determine the anisotropic characteristics of wingtip vortex wandering. The secondary perturbation mode of group A has the characteristic that the fluctuation of flow velocity is larger than that of transverse velocity， while the secondary perturbation mode of group S has a higher tangential wavenumber and range of action. The flow direction evolution law of different perturbation modes is different. The main perturbation mode and P group perturbation mode of the wing tip vortex rotate along the flow direction， with the perturbation amplitude gradually magnified with the flow direction. Group A secondary perturbation mode will slowly increase the perturbation amplitude with the development of the flow direction. Group S secondary perturbation mode will gradually cover the whole vortex core with the flow direction， and the perturbation passing through the vortex core will be further magnified with the flow direction. The evolution law of perturbation energy of different wing tip vortex perturbation modes with flow direction is described. S secondary perturbation modes have higher tangential wavenumber characteristics and meanwhile higher perturbation energy growth， meaning that using S-group secondary perturbation modes to guide the active control of the wing tip vortex is the most potential strategy.