Designing a device with high sensitivity,which is used to measure the concentration of soot based on optical backscattering. It is not only characterized by real time,on-line and continuous measurements but also because its price is cheap and it is easy to install. It achieved the relation between optical intensity and concentration of soot by theoretic analysis and measured optical intensity in different concentration by using the device which was designed by the researchars of this paper. It testified theoretic validity and achieved the curve which describe the relation between concentration and signal intensity. The experimental result shows that there is a good linear relation between back-scattering lighe intensity and concentration under the condition of lower concentration.