Laser-induced breakdown spectroscopy is a simple and fast element detection method which is applicable for liquid samples. In order to reduce the quenching effect and hydrodynamic instability of the liquid sample, the liquid-to-solid phase conversion is often performed. After phase conversion, it is difficult to confine the solute to a micrometer-scale region on the substrate, so the excited laser beam with large beam spot and small longitudinal focal length is preferred. Therefore, the simultaneous spatial and temporal focusing laser is expected to be more suitable for liquid sample detection. To evaluate the effect of the spatiotemporal focusing, a silicon wafer coated with 400-nm-thick Cr film is used as the sample. Experimental results show that the spatiotemporal focusing laser can excite only Cr film, which cannot be achieved by the spatial focusing laser. Using the spatiotemporal focusing laser, the limit of detection of Pb in water is down to 12.5 fg. Contrastingly, using the spatial focusing laser, the elements in the substrate are excited which makes the detection of the trace Pb element in liquid samples become impossible. The method presented in this study may find important applications in water pollution control, urine and blood tests.