A novel multistatic multiple-input multiple-output (MIMO) integrated sensing and communication (ISAC) system in cellular networks is proposed. It can make use of widespread base stations (BSs) to perform cooperative sensing in wide area. This system is important since the deployment of sensing function can be achieved based on the existing mobile communication networks at a low cost. In this system, communication reference signals transmitted from the central BS are received by the neighboring BSs. Joint data processing is then performed to estimate the position and velocity of sensing objects. In addition, we analyze key issues in multistatic ISAC system in cellular networks including synchronization, uplink/downlink frame structure, interference and sensing & communication coexistence. Simulation results shows that the proposed multistatic system can reduce interference power by over 10 dBm for sensing function and improve the object estimation accuracy when compared with monostatic system, demonstrating the effectiveness and promise of implementing multi static ISAC in the upcoming fifth generation advanced (5G-A) and sixth generation (6G) mobile networks.