Based on the data of cores, well logging and casting thin sections, the spatial structure of calcareous interlayer in Wenchang A Oilfield, western Pearl River Mouth Basin, are analyzed by using seismic waveform indication inversion of SMI software. On this basis, the influence of calcareous interlayer on reservoir development effect is discussed in combination with structural location of development wells, reservoir physical properties and productivity. It is shown that the P-impedance can distinguish the calcareous layer obviously. The accuracy of seismic waveform indication inversion based on the P-impedance is so high that the 0.5m calcareous interlayer can be identified. Three sets of calcareous layers are developed in ZJ2Ⅰ-1 sand group of the study area, which are divided into top, middle and bottom calcareous layer according to their vertical position. The top and bottom calcareous layer are dominated by bioclastic limestone, it is formed in the foreshore sand bar at the top of ultra-short term reverse cycle with high frequency oscillation of sea level, and their plane extension scale is large. The middle calcareous layer is dominated by calcareous sandstone, which is formed in the middle and top of the ultra-short term positive cycle with rising base level. The formation of medium calcium layer is related to differential diagenesis with limited distribution and poor lateral continuity. Calcareous interlayer indirectly affects the development effect by affecting the water content. However, when the bottom calcareous layer is missing or there is a skylight, if the physical property is good, the production capacity can be improved by liquid extraction measures, and if the physical property is poor, the development effect is poor. The new understanding breaks through the traditional view of "if there is a skylight in the bottom calcium layer, the productivity is low", and can provide a basis for tapping the potential of the oilfield.