Magnetic fields are believed to play an important role in controlling the stability and contraction of molecular cloud cores. In present study, magnetic fields of cold prestellar cores are mapped based on wide-field near-infrared polarimetric observations of thousands of background stars. A distinct "hourglass-shaped" magnetic field is identified toward prestellar cores, as the first observational evidence of magnetic field structure distorted by the gravitationally contracting medium. Our findings on the geometry of magnetic fields as well as the mass-to-magnetic flux ratio will be discussed in the talk.