The 21CMA array is a low-frequency radio interferometer built in a remote mountain valley named after the Mongolian word "Urastai", which means aspen trees, in the northwest province of Xinjiang, China. The array consists of 81 antenna pods that are distributed in the north-south (4 km) and east-west (3-6 km) baselines, respectively, and each pod is an assembly of 127 logarithmic periodic antennas. The array is operated in 50-200 MHz (including the FM radio band), which will enable the scientists to detect the redshifted 21 cm emission from the reionization era to probe when and how the first objects in the universe were formed. Separated as parts of contaminating foregrounds over the reionization signals, the behaviors of quasars and galaxy clusters in the low-frequency radio band will also be assessed.