In this four tank system, there are two loops connected with each other. The performance of control algorithm often relies on the mathematical model. The example shows how to identify the model of NO.1&2 tank represented by LI_001 and LI_002 respectively using Least Square and M-series signal. As the screenshot shows, the upmost line represent the M-series signal given to frequency inverter as input, other lines show the water levels of four tanks. We use about 600 data to meet the PE condition and get the first-order to sixth order model. Besides, comparison between measured data and simulated outcomes are given.