Marine sequences in Lower Yangtze area experienced extensive tectonic transformation in Indo-Yanshan period. Preservation condition is one of the key factors controlling accumulation of marine petroleum. The Silurian cap rock played a decisive role in the preservation or destruction of the Paleozoic primary oil and gas reservoirs. Based on the experimental data from cores of 7 wells meeting the Silurian cap rock and 3 outcrops with a total number of 33 samples, the present-day sealing capacity of the Silurian cap rock was evaluated. Based on the mathematic models of porosity-capillary pressure and permeability-capillary pressure histories and the parameter of over consolidation ratio（OCR）, the dynamic evolution of sealing capacity of the Silurian cap rock was reconstructed. The results show that the Silurian cap rock is of low porosity, low permeability and high capillary pressure and therefore with good sealing capacity in present day. At the burial stage, the capillary pressure reached as high enough as can seal high pressure or even ultrahigh pressure gas reservoirs, but in the early Cretaceous period the capillary pressure of the Silurian cap rocks in Jurong area significantly decreased and their OCR values exceed the threshold value of brittle fracture. Therefore, the preservation condition of the Lower Paleozoic experienced serious damage. Comparatively, the degree of damage of preservation condition is relatively weak in Changzhou and Taixin areas.