A design of a portable low-cost measuring sensor for weak magnetic fields based on single-chip microcomputer which was used in experimental education was presented.The hardware structure of the instrument as well as software realizing method were demonstrated.A measurement of geomagnetic field as a standard reference was conducted.Through measuring the distribution of geomagnetic field around a iron sphere and comparing the experimental result with result of theoretical calculation

the design of this instrument was proved to be highly accurate and stable.