Amaranth is widely added to food and can cause many adverse health effects when it is excessively consumed. Therefore, the monitoring of amaranth is quite important. Herein, an electrochemical sensor for the sensitive and rapid detection of amaranth was reported using multiwall carbon nanotube (MWNT) as the sensing film. Due to the large surface area and high accumulation efficiency, the MWNT sensor showed a strong enhancement effect on the oxidation of amaranth, and greatly increased the current signal. The detection conditions such as pH value, amount of MWNT, accumulation potential and time were optimized. The linear range is from 40 nM to 0.8 μM, and the limit of detection is 35 nM. Finally, the new sensor was successfully employed to detect amaranth in soft drinks, and the results were tested by high-performance liquid chromatography.