Embodiments of the invention provide a power transmission line online monitoring information acquisition method, device and system. The method includes the steps of sending an acquisition instruction; receiving one or more data packets returned by a sensor; if each data packet contains a data packet header, deleting characters located before the data packet header, and if each data packet contains a data packet tail, deleting characters located after the data packet tail; determining whether the remaining characters in each data packet are preset valid characters, and if not, deleting the characters; assembling the remaining characters in all the data packets to obtain a reassembled data packet; verifying the reassembled data packet according to a preset verification rule; and if the verification fails, discarding the reassembled data packet and resending an acquisition instruction. According to the invention, the problem of data packet bit errors caused by electromagnetic interference and the like is effectively solved, the reliability of the power transmission line online monitoring system is improved, and the advantages of low cost, good maintenance and the like are gained.