Abstract: The Perfectly Matched Layer (PML) is an effective absorbing boundary and has been widely used in acoustic simulation. In order to develop an absorbing boundary condition for numerical simulation based on the second-order acoustic wave equation, an Unsplit PML algorithm is proposed. Firstly, frequency-domain expression of this method is formulated based on the complex stretching coordinate schema. Then, its time-domain expression is derived by formulating auxiliary differential equations. Finally, relative theoretical analysis and numerical simulation are carried out, the results of which demonstrate that compared with the existing Split PMLs, the proposed method has the same absorbing efficiency and can reduce storage greatly. It can also increase calculation efficiency with easier implementation.