Breath isoprene is an endogenous metabolite whose concentration is related to human blood cholesterol level. However, numerous factors are influencing human breath. It is necessary to conduct effective breath analysis in selected specific populations (access large breath data with real-time, online, high sensitivity, high selectivity, and high accuracy). Cavity ringdown spectroscopy (CRDS) is a highly sensitive, highly stable, and highly selective spectral technique. In this paper, a breath isoprene analyzer based on CRDS was constructed considering the single-wavelength integrated semiconductor UV laser currently on the market. The breath isoprene analyzer mainly comprised of laser source, vacuum cavity, photodetector, and data acquisition. The fitting result showed good linearity indicating that the ringdown signal obtained by this analyzer was close to the single exponential decay (R²=0.998 39), and obeyed Lambert-Beer’s law. Effects of different average times on ringdown signal stability were evaluated. Considering the stability of the ringdown signal and the response time of the analyzer comprehensively, 128 times was used as the average time during the experiment. Subsequently, the performance of the breath isoprene analyzer was investigated. Ringdown times of 16 min under vacuum were measured continuously. The repeatability and response of the breath isoprene analyzer were examined. We measured the ringdown times of standard isoprene gas with different density of particle (10×10^-9, 30×10^-9, 50×10^-9, 100×10^-9, 200×10^-9) to calculate the linearity of the analyzer. At last, the spectral interference (NO, N₂O and acetone) at 224 nm was discussed. The experiment shows that: the ringdown breath analyzer has high sensitivity (limit of detection is 0.49×10^-9), good repeatability, stability (0.48%), near real-time response (1 datum per second) and good linearity (R²=0.993 13). The improved LoD in this paper is about 1/1 000 of the current LoD. The portable breath isoprene analyzer based on CRDS could realize the effective analysis of isoprene in actual human breath.