Particle counter of extinction type is used mainly to detect the particulate contamination of larger sizes, and widely applied in contamination control measurement and monitor, whereas it is unable to detect smaller sizes particulate, the reason is discussed in this paper. Based on Mie theory, extinction coefficient is calculated, at the same time, the representative structure of this type sensor is analyzed. Numerous calculation and analysis showed that it is due to the ambiguity relationship of extinction coefficient versus particulate sizes less than 2 μm and unreasonable design. An improved design of the sensor was proposed, which employs a specially designed detecting lens unit to collect more scattered light and a monitoring device to control the light source, the lens of collecting scattering light is arranged at 45 degree, which can lead the good monodromy for light energy versus particulate sizes and less interference due to particle refractive index. Comparative experiments showed that the lower limit of the measuring size range could be extended by the new sensor design. The new sensor has a good detect ability for particle sizes less than 2 μm and lower repetition errors, and furthermore, it still has a high level for detecting larger sizes particle, it suits for more application of contamination degree monitoring.