The invention provides a method for monitoring the Ploy (A) tail length of mRNA and application, and relates to the technical field of biology. The provided method for monitoring the Ploy (A) tail length of the mRNA comprises the steps that in an mRNA tailing reaction, the ATP consumption amount and/or the PPi generation amount is measured to obtain the Ploy (A) tailing length of the mRNA. The method is suitable for various types of mRNAs, wide in universality and capable of effectively replacing complex detection methods such as capillary electrophoresis adopted in the prior art for quickly and effectively detecting the tailing length of the mRNA, so that detection is simpler and more efficient, in addition, a relationship between the ATP consumption amount and the tailing length of the mRNA is utilized for monitoring the Ploy (A) tail length of the mRNA, and a result is more visual and more accurate. In addition, detection for the ATP consumption amount does not need expensive instruments, and the detection cost is low.