A package-board co-design method was applied for a Narrowband Internet-of-Things (NB-IoT) SiP module. The electromagnetic interference (EMI) generated by the module was studied by improving the transmission quality of radio frequency (RF) signal. The SiP models of the initial design and the optimized design were simulated separately to show that the optimized design significantly increased effective transmission power of the RF signal and suppressed near-field electromagnetic radiation intensity to a certain extent. In addition, the optimized design model was verified by measurement. The measured results show good agreement with the simulated ones and demonstrate that the package-board co-design method can improve the electromagnetic compatibility (EMC) of NB-IoT applications.