Avalanche photodiode (APD) has a wide range of applications for time- correlated single- photon counting in quantum secure communication, quantum cryptography, and other quantum optics fields. However, during APD operation, incident photons in the absorption layer create carriers, which then multiply exsponentially. As each carrier goes through the P-N junction node there is a certain probability that it will cause the emission of another photon. Under certain conditions this newly emitted photon can generate crosstalk in the other avalanche photodiode, thus affecting the time-correlated single-photon counting results when detecting single photons in the Geiger mode. Through experimentally reproducing the phenomenon of relatively high contrast crosstalk interference we analyze the factors influencing the crosstalk peak, including peak spacing and shape. A scheme using optical isolation is proposed to avoid the crosstalk, and its feasibility is verified experimentally.