The invention discloses a packet loss prevention key pre-negotiation method based on an IPSec VPN (Internet Protocol Security Virtual Private Network). The method belongs to the technical field of communication networks, and comprises the following steps: setting a pre-negotiation mark, reversing the value of the pre-negotiation mark once every time the pre-negotiation is carried out, and a pre-negotiating device A and a pre-negotiating device B to generate an SPI (Serial Peripheral Interface) and a symmetric key when the negotiation lifetime is not ended; the device A calculates a logic tunnel number needing to be stored according to the value of the current pre-negotiation mark; and the device A performs hash processing on the decryption key stored in the channel corresponding to the logic tunnel number and then puts the decryption key into a queue, so that after the negotiation lifetime expires, the device B obtains the key from the corresponding logic tunnel number according to theSPI to realize decryption. When the lifetime is expired, no matter whether the device A selects a new key or an old key for encryption, the device B finds the corresponding logic tunnel number according to the SPI after receiving the new key or the old key, and finds the key in the hash queue from the logic tunnel for decryption, thereby ensuring that data packets going back and forth after the lifetime of the key is expired cannot be lost.