The invention discloses a thermal performance on-line monitoring system for a wet cooling column and a method thereof. The thermal performance on-line monitoring system includes: an environment parameter collection device, which is used for collecting information of environment above the wet cooling tower; and a column interior thermal performance parameter collection device, which is used for collecting the column interior thermal performance parameters of all measurement points, wherein the measurement points for temperature of air above a dehydrator, speed of air above the dehydrator, temperature of water above a pack, and temperature of water under the pack are respectively arranged for at least two layers according to different elevation, in each layer, the measurement points are arranged along the radius in a cross-shaped manner, and the number of the measurement points in each layer is determined on the basis of equivalent plane ring principle. Measurement points for temperatureof a water collection tank are arranged on the surface of a water collection tank according to the equivalent plane ring principle. Measurement points for inlet temperature of circulating water are at least two in number and are symmetrically and uniformly arranged in a water distribution silo; measurement points for outlet temperature of the circulating water are at least two in number and are symmetrically and uniformly arranged at a water outlet of the water collection tank.