In 5G networks, user data will assemble larger packets through ultra-dense base stations. When these packets are amplified by erbium-doped fiber amplifier (EDFA) in the dense wavelength division multiplexing (DWDM) system using an asynchronous switching system, interchannel crosstalk will occur and the bit error rate (BER) of the data packets will be deteriorated. Based on the measurement of gain recovery time of EDFA, the BER deterioration is studied in depth. The calculation formula of BER caused by crosstalk is derived, and the factors that influence BER are discussed. An experimental device of large data packet crosstalk is set up. Both theoretical and experimental results show that the greater the packet length and optical power of the data packet in one channel, the greater the effect on the BER of the data packet in another channel. When the packet length is less than 5 μs, the effect of crosstalk on the BER is negligible.