This paper investigated the influences of K-Cs antisite, K-Sb antisite, and Cs-Sb antisite on the electronic structure properties and optical properties of K-Cs-Sb photocathodes by the first-principles method based on density functional theory. Besides, we analyzed the electronic structure properties such as energy band structure, density of states, and formation energy, and the optical properties such as refractive index, extinction coefficient, and absorption coefficient associated with different defect models. The calculation results regarding the electronic structure properties show that the K-Cs-Sb antisite defect models with Sb occupied by excess alkali metal have indirect band gap structures and take on the nature of an n-type semiconductor. In contrast, the K-Cs antisite defect models and the K-Cs-Sb antisite defect models with alkali metal occupied by excess Sb exhibit the nature of a p-type semiconductor. The antisite defect model of K₂Cs_0.75Sb_1.25 is easier to form and more stable than its counterparts. In addition, the calculation results regarding optical properties demonstrate that the absorption coefficient peak shifts to the lower energy position due to excess alkali metal, while the situation is just opposite for excess Sb metal. In the energy range (i.e., 2.4-3.2 eV) of the neutrino-scintillator interaction, K₂Cs_0.75Sb_1.25, which has the largest absorption coefficient and the smallest refractive index, is a better choice as photoemission materials than traditional K₂CsSb.