The invention discloses a 
quantum key agreement protocol based on GHZ state, comprising the following steps that: step 1, 
Alice and Bob randomly generate respective classical keys; step 2, Alice prepares the GHZ state and divides all particles into sequences, inserts 
decoy photons into one of the sequences and then transmits the sequence to Bob; step 3, Bob measures the 
decoy photons, Alice calculates an error rate, if the error rate is low, a step 4 is executed, otherwise, the step 2 is executed again; step 4, 
Alice and Bob respectively perform measurement and obtain the measurement result of each other; step 5, Alice executes 
unitary transformation and obtains a new sequence, and Alice transits the sequence with the inserted 
decoy photons to Bob; step 6, Bob measures the decoy photons, and Alice calculates the error rate, if the error rate is low, a step 7 is executed, and otherwise, the step 2 is executed again; step 7, Alice calculates a shared key of both sides; step 8, Bob generates the shared key. The 
quantum key agreement protocol based on GHZ state can 
resist participant 
attack, outside 
attack and 
Trojan horse attack. The 
quantum key agreement protocol based on GHZ state is safe in both a noiseless 
quantum channel and a quantum noisy channel. Moreover, quantum bit efficiency of the quantum key agreement protocol based on GHZ state is higher than the existing protocols.