Expression of a high concentration of H2S is closely related to the formation of colon cancer tumor. However, up to now, only few H2S triggered theranostics agents for colon cancer have been reported. Herein, a turn on theranostics agent was developed via utilizing the in situ reaction of Cu2O and endogenous H2S at colon tumor sites. Based on the In vitro and in vivo experiments, excellent photoacoustic imaging and photothermal therapy have both been confirmed by this in situ reaction activated colon cancer theranostics. Our work established a simple and efficient strategy for both diagnosis and treatment of colon cancer with a novel trigger mechanism, which provides a new route for colon cancer theranostics based on the in situ reactions at the tumor sites.