In order to study the mechanism of the second harmonic generation (SHG) effect in borophosphates, two large ultraviolet nonlinear optical crystals, SrBPO5 and BaBPO5, have been successfully obtained using a top-seeded solution growth method. Optical and thermal properties have been measured, and SrBPO5 and BaBPO5 are shown to display large experimental gaps (6.44 eV and 6.14 eV, respectively) and moderate SHG coefficients (0.57 and 0.43 times that of KH2PO4). More importantly, based on a further understanding of the relationship between nonlinear optical properties and crystal structure within density functional theory (DFT), we have demonstrated that BO4 groups and metal cations play an essential role in the SHG effect of SrBPO5 and BaBPO5, but not PO4 groups as anticipated. Indeed, PO4 groups play a counteractive role in BaBPO5 due to the indirect influence of metal cations.