Abstract: An 11 MeV H－ compact cyclotron used for medical radioactive isotope production is under construction in Institute of Fluid Physics, CAEP. The cyclotron magnet adopts the design of small valley gaps and coulee structure which can provide high average magnetic field and strong focus ability. To achieve 510-4 measuring accuracy, a magnetic field mapping system has been developed. After iterative correction using field measurement data, the total phase excursion of the cyclotron is within 9 and the first harmonic is less than 10-3 T, which are all acceptable. Furthermore, the beam testing declares the successful construction of the cyclotron magnet. Besides, some magnetic field influence factors were discussed, including the magnetic field distortion and measurement error.