A numerical method, in which the average bond energy is considered as a reference level for determining the valence-band offset (VBO) in strained-layer superlattices (SLS's), is suggested and tested by performing ab initio electronic-structure calculations for the SLS's (InP${)}_{\mathit{n}}$/(InAs${)}_{\mathit{n}}$(001), (n=1,3,5), (GaP${)}_{1}$/(GaAs${)}_{1}$(001), (AlP${)}_{1}$/(AlAs${)}_{1}$(001), and (AlP${)}_{1}$/(InP${)}_{1}$(001). Based on this method, the VBO's of the SLS's composed of any two of the six compounds InP, InAs, GaP, GaAs, AlAs, and AlP under arbitrary elastic strain are determined systematically, and their strain-induced effects are discussed. Good agreement is found between the present results and the available experimental data.