Structure engineering presents unique opportunities in materials science field, including material design and modification. Herein, we applied structure engineering to double-sublayer hexagonal C₂P₂ monolayers so as to form two novel non-Janus structures and two new Janus structures. Based on first-principles calculations, the stability, electronic, optical, and photocatalytic properties of the C₂P₂ monolayers, including the two discovered structures and four new C₂P₂ monolayers, have been investigated. The results showed that these C₂P₂ monolayers are highly stable in energetics, dynamics, and thermodynamics. We also found that counterrotating 60° between the top and bottom sublayers could make the C₂P₂ monolayers become more stable. The calculations of the project band structures indicated that the new C₂P₂ monolayers were semiconductors with indirect band gaps ranging from 1.02 eV to 2.62 eV. Meanwhile, it was also suggested that the distributions of VBM and CBM in the two Janus C₂P₂ monolayers were out-of-plane due to their internal electric fields. Moreover, the carrier mobility of the C₂P₂ monolayers was anisotropic between an armchair and zigzag direction and quite high (reaching 10³ cm² V⁻¹ s⁻¹) in the zigzag direction. Additionally, all the C₂P₂ monolayers had large exciton binding energies (∼1.0 eV) and considerable absorption in the visible-light region. Furthermore, except for the CP-3 monolayer, all the C₂P₂ monolayers, including CP-1, CP-2, CP-4, CP-5, and CP-6, have great potential for metal-free visible-light photocatalytic water splitting. Our calculations reveal that structure engineering is particularly applicable to multi-sublayer two-dimensional materials for discovering new members and tuning their properties.