Infrared (IR) spectroscopy is an important tool for the qualitative analysis of liquid matters. IR spectroscopy of liquid samples is commonly prepared by liquid film method and tested by transmission (TR) method, while the salt window required is expensive, prone to force or moisture cracking. The incomplete surface cleaning or scratches can easily cause test interference; moreover, installing the demountable liquid cells is troublesome, and mixing the trapped air into the sample will also cause test interference. In this paper, a simple method for measuring IR spectra of liquid substances is studied. The differences between the improved TR method, samples prepared by smearing method of directly coating liquid onto a single-use compressed potassium bromide (KBr) disc, and the attenuated total reflection (ATR) method in IR spectra of liquid substances are compared. 6 kinds of liquid reagents with different volatility, hygroscopicity and corrosivity were selected as the research objects, and their IR spectra were tested by both the improved TR method and ATR method. The IR spectra measured by the two methods were compared with those in the SDBS spectral database. The influence of scanning times and resolution on ATR spectra was also studied. The results show that the two methods are accurate in the IR qualitative analysis of liquid samples. The improved TR method simplifies the sample preparation process, avoids the problem of cleaning salt window, and reduces the cost, but water interference is still difficult to avoid. In contrast, the ATR method requires no sample preparation, is more simple, convenient and faster, and the interference of water is negligible. Although the overall intensity and fineness of the spectra tested by the ATR method are not as good as tested the TR method, the high-quality spectra can be obtained by improving the resolution and increasing the scanning times. The amount of liquid should be increased when using the improved TR method and ATR method for volatile liquids. The improved TR method is recommended for strongly acidic and/or corrosive liquids. For hygroscopic liquids, the spectra measured by the ATR method are easier to analyze. In contrast, except for strongly acidic and/or corrosive liquids, the other liquid substances can be rapidly and accurately tested by the ATR method.