An integration unit for electric energy quality monitoring comprises a clock pulse synchronizing module M1, a sampling pulse output module M2, a data receiving and decoding module M3, an interpolation moment calculating module M4, an interpolation data processing module M5 and an assembly line control module M6 which are implemented through field programmable gate array (FPGA). The invention further provides a synchronous sampling method of the integration unit. By combining the synchronous pulse method with the interpolation method, the problem about low synchronizing accuracy caused by the use of single synchronizing method is effectively solved. Secondly, the problem about instable sampling accuracy during long-term operating is solved by adopting the self-adaption adjustment interpolation moment algorithm, and the problem about low accuracy of ultraharmonics is solved by adopting the third-order newton interpolation algorithm. In addition, the integral system adopts assembly line control mode and is guaranteed to stably operate at high sampling speed. The integration unit creates conditions for an electric energy quality monitoring device to access a digital substation.