We design and investigate a triple-band plasmonic metamaterial absorber (PMA) for sensor application. The underlying mechanism is investigated theoretically and numerically. Three characteristic absorption peaks are demonstrated to be induced by different plasmonic modes which lead to different responses for the plasmonic sensor. These modes show great improvement for the sensitivity and accuracy of the plasmonic sensors. This triple-band plasmonic metamaterial optical absorber has great potential to improve the performance in practical applications.