Trapping potential of a Z-trap is calculated with analytic and numeric methods. It's found that when a trapping center is a little bit far from the chip surface (the distance is about one order of the half length of the Z-wire central part), the trapping depth is not approximately eqsual to the potential By created by a bias magnetic field, the potential energy at the trapping center should be subtracted from the potential By created by the bias field. On the other hand, if an atom cloud is compressed to a certain extent by increasing By, the trapping depth will be decreased rather than increased. The preparation of the Z-trap in an atom chip, the experimental setup, and the experimental procedure for trapping neutral 87Rb atoms is also introduced. At last we obtained 2×106 87Rb atoms trapped in the Z-trap.