Common signals should be transmitted omnidirectionally to guarantee cell-wide coverage in massive MIMO systems under celluar communication units. Space-Time Block Code (STBC) has received widespread application in MIMO systems, in this paper, we focus on the omnidirectional STBC in massive MIMO downlink, which provides a new thinking for reducing the burden of downlink pilot overhead. We firstly design an omnidirectional precoding (OP) matrix for 2*2 Alamouti Code, which guarantees equal receiving power for user terminals in all spatial directions at any instant time as well as equal instantaneous power for each transmitting antenna. To obtain diversity order of four, OP matrices utilizing $4*4$ STBC and OSTBC Code are also proposed, respectively. What's more, the condition of OP matrices is rewritten considering constant angle domain, and several examples of OP matrices are proposed. We analyse the system performance utilizing omnidirectional STBC in four aspects: bit error rate, peak-to-average ratio (PAPR), channel capacity and outage probability. Simulation results show that one proposed design can 1) reduce bit error bit; 2) maximize channel capacity; 3) preserve the PAPR of transmitted signals.