A method for data hand-shaking based on EtherCAT protocol, implemented by an EtherCAT slave device having an EtherCAT slave controller, ESC, and comprising: a) receiving a data packet from a previous data processing device through a first receiving route, wherein the previous data processing device is an EtherCAT master device or another EtherCAT slave device; b) obtaining self-related data of the EtherCAT slave device from the data packet or inserting feedback data of the EtherCAT slave device to the data packet by the ESC for generating a processed data packet; c) delivering the processed data packet to a next slave device through a first transmission route, wherein the next slave device is another EtherCAT slave device, and the previous data processing device, the EtherCAT slave device, and the next slave device are in series connection with each other; d) receiving a returned packet from the next slave device through a second receiving route, and delivering the returned packet to the previous data processing device through a second transmission route; e) performing a packet diversion procedure to at least one of the data packet, the processed data packet and the returned packet for generating a backup packet, wherein at least a part of the content of the backup packet is identical to the content of the data packet, the processed data packet or the returned packet; and f) analyzing the backup packet by the ESC for filtering and obtaining a related data from the backup packet, wherein the related data is a designated application data with respect to one or more other EtherCAT slave devices designated to be synchronized with the EtherCAT slave device; wherein the step a) to the step f) are completed within one single communication cycle. a) receiving a data packet from a previous data processing device through a first receiving route, wherein the previous data processing device is an EtherCAT master device or another EtherCAT slave device; b) obtaining self-related data of the EtherCAT slave device from the data packet or inserting feedback data of the EtherCAT slave device to the data packet by the ESC for generating a processed data packet; c) delivering the processed data packet to a next slave device through a first transmission route, wherein the next slave device is another EtherCAT slave device, and the previous data processing device, the EtherCAT slave device, and the next slave device are in series connection with each other; d) receiving a returned packet from the next slave device through a second receiving route, and delivering the returned packet to the previous data processing device through a second transmission route; e) performing a packet diversion procedure to at least one of the data packet, the processed data packet and the returned packet for generating a backup packet, wherein at least a part of the content of the backup packet is identical to the content of the data packet, the processed data packet or the returned packet; and f) analyzing the backup packet by the ESC for filtering and obtaining a related data from the backup packet, wherein the related data is a designated application data with respect to one or more other EtherCAT slave devices designated to be synchronized with the EtherCAT slave device;