The interstellar magnetic field is essential in various astrophysical phenomena and processes. Pulsar halos are a recently discovered class of TeV $\gamma$-ray sources formed by escaping electrons/positrons from pulsars. The morphology of the halo is regulated by the diffusion of those escaping particles and hence carries information of the interstellar magnetic field. We suggest that the morphology of TeV pulsar halos can be used as a novel probe of the properties of the interstellar magnetic field around the pulsar, such as the Alfvénic Mach number and the mean direction. We establish a theoretical relation between these quantities and the observational features of the halo’s morphology based on the anisotropic diffusion model and show how x-ray observations of the pulsar halos can further improve the diagnosis of the magnetic field.

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• Cosmic ray propagation
• Diffusion
• Gamma ray astronomy
• Turbulence
• Interstellar medium