对标准砂和风干枯土的击实试样进行了真三轴应力应变关系试验研究，发现当某一方向给土体施加压应力增量而其侧向应力保持不变时，侧向变形未必都是膨胀的，也有可能是压缩的。即泊松比未必总是正值，而有时可能为负。这是一个值得重视的有异于经典力学理论的现象，本文研究了这一现象出现的规律，并从机理上分析了其发生的必然性和存在的条件。 A true triaxial stress-strain relationship study was performed on compacted specimens of standard sands and air-dry soils. It was found that when a certain direction exerts a compressive stress increment on the soil and its lateral stress remains unchanged, the lateral deformation may not be uniform. It is inflated and it may be compressed. That is, the Poisson’s ratio may not always be a positive value, but it may be negative sometimes. This is a phenomenon that is worthy of attention and is different from classical mechanics theory. This paper studies the law of this phenomenon, and analyzes its inevitability and conditions.