The invention relates to a method for synthesizing acetic acid through low-pressure methanol carbonylation and a device thereof, belonging to the field of chemical technology. The invention comprises the following steps: firstly, carrying out reaction between methanol and carbon monoxide in a reactor containing a reaction solution so as to generate the acetic acid; feeding the reaction solution into a catalyst recovery tower to recover a catalyst through flash vaporization and washing; feeding flash vaporization gas phase into a rectifying tower, and obtaining finished acetic acid through rectification; condensing fractions of the top of the rectifying tower and then dividing the fractions into light phase and heavy phase, wherein a part of the light phase flows back to the upper part of the rectifying tower, and the remnant light phase and heavy phase flow back to the reactor to continuously take part in reaction. In the invention, the prior functions executed by a lightness-removingtower and a dehydrating tower are merged to be completed in the rectifying tower so that the process route of acetic acid preparation is simplified, the equipment investment of the device is reduced,the difficulty of operation control is reduced at the same time, consumption of the device can be effectively reduced, and the production cost is reduced.