[Objective] We focused on finding out the active area of catalytic domain of endolysin Ly7917 against Streptococcus suis, consequently helpful for Ly7917 modification. [Methods] A group peptides truncated from N-terminal of Ly7917 or C-terminal of LyCHAP were expressed and compared with original Ly7917 and LyCHAP of the catalytic activity by plate lysis assay and turbidity decrease assay, even the situation with addition of Ca2+. [Results] LyCHAP showed excellent catalytic activity as well as the full-length Ly7917. Truncated Ly7917 from N-terminal showed no catalytic activity. With the assistance of Ca2+, LyCHAP without 20 amino acids at C-terminal (LyCHAP1?130) could achieve the maximum catalytic effect even better than full-length LyCHAP. [Conclusion] Ca2+-dependent LyCHAP1?130 could be the potential alternative of Ly7917 for further clinic trials.