Recent experiments highlight second-order nonlinear optics (NLO), including the bulk photovoltaic effect (BPVE) and second-harmonic generation (SHG), in weak interlayer coupling materials, as the photoelectric conversion is enhanced with layer numbers [Nature (London) 613, 53 (2023)]. Here, based on first-principles calculations, we investigate the BPVE and SHG in $\mathrm{Sn}{\mathrm{P}}_2\mathrm{S}{\mathrm{e}}_6$. The small interlayer coupling energy and minor band-gap variation with layer number indicate its weak interlayer coupling. The BPVE and SHG responses increase with layer number, regardless of the parity of the layer number. Moreover, the interlayer sliding has a negligible influence on the BPVE, which guarantees that the BPVE is robust against structural deformation. The parity-independent, sliding-insensitive BPVE, along with the accumulation effect with layer number of BPVE and SHG in $\mathrm{Sn}{\mathrm{P}}_2\mathrm{S}{\mathrm{e}}_6$, indicates its potential applications in NLO devices.

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• Second order nonlinear optical processes
• Shift current
• First-principles calculations