Abstract: Through the field vane strengths S_uFV of 19 projects involving two sedimentary facies of coastal plain and lacustrine plain in Shanghai and combined with the worldwide research findings, the overconsolidation ratio (OCR) and its geological causes of three Shanghai soft clay layers are analyzed. The OCR is calculated according to strength ratio S_uFV/ ( is the effective overburden pressure), and then its variation with depth is given. The results show that the Shanghai soft clay is overconsolidated in general. The OCR of shallow layers is larger and more discrete (from 1.3 to 5.0). The OCR decreases rapidly with the increase of depth and almost maintains a stable value (1.6 in averages) after 13 m deep. The OCR of Shanghai soft clay is significantly underestimated according to the laboratory compression tests in the past. Based on the model given by Hanzawa and Tanaka, the strength and geological causes of overconsolidation of Shanghai soft clay are analyzed. It is conformed that for the shallow layers of the clay the main cause is the cement effect varying in a wide range, and for deep layers the main cause is the secondary compression effect being in a relatively stable range. The contribution of normal compression, cement effect and secondary compression effect to the total strength of the soft clay is also given.