Abstract: As a special form of pile foundation, the cement mixed-in-place piles are widely applied in weak so i1 region．These piles are formed with cement and soi1 mixed-in-place with a ratio of cement to soi1 about 12％-15％. They usually have a diameter about 500-600mm and a1ength from 8-18m. Their density is about 2.0T/m3 and the velocity of longitudinal wave is generally from 1 000m/s to 2 220m/s. Because the velocity of longitudinal wave in cement mixed-in-place piles is far lower than the velocity in the concrete piles , the ratio of length to diameter is generally smaller and their bodies are relatively inhomogeneous, whether the reflection wave method can be used to examine the quality of cement mixed-in-place piles is still open to question though the method has been commonly applied to cheek the concrete piles. In this paper, we will analyze the nature of reflection wave method and suggest that the difference of wave impedance between pile and soil and the relation between resolution and the wave length &re the keys to the application of this method. The wave impedance in cement mixed-in-place piles is much higher than that in the soil around and under the pile. The wave 1ength in cement mixed-in-place piles is much smaller than 1ength of the pile. Because the wave theory has demonstrated that the reflection wave can distinguish the object with a size bigger than 1/4 wave length, the reflection wave method therefore coil be used to examine cement mixed, in place piles. We wil1 also discuss the meaning of L/D and its relation to 1-D wave theory and put forward some new point of view about pile integrity in this paper. Based on theoretical study and practical experiments, we believe that the reflection wave method is effective in examining the cement mixed-in-place piles. However, comparing with examining the concrete piles, the following peculiar techniques should be enphasized in practice: a. The obvious 1eve1 and extent of reflection signal of pile bottom are vita1 features that are related to the quality of cement mixed-in-place piles. If pile body is relatively homogenous and the strength of pile body is relatively high, the reflection signa1 of pile bottom is clear. b. W hen the piles are examined after 28 days since their formation, the wave velocity of 1 O00m/s or higher could be considered as the standard for examining the strength of piles even if the reflection signa1 of pile bottoms are not very obvious. c. Between the signal of pile top and the reflection signal of pile bottom, the curve shape may change because the pile body is inhomogeneous and the wave impedance of pile body is different. Even so, if the reflection signal of pile bottom is very clear, the wave velocity can reach the standard and the pile body is not very inhomogeneous, the pile is also passable.