A highly efficient cyclopropanation of diazo compounds and electron-deficient alkenes under metal- and catalyst-free conditions was developed. The use of water as the sole solvent dramatically increased the efficiency of the reaction. This on-water synthesis approach featured numerous advantages, including high yield, broad substrate scope and environmentally friendly conditions. Moreover, a simple substrate-engineering strategy was used in the reaction system to realise the unprecedented diastereodivergent synthesis of functionalised cyclopropane derivatives with remarkable stereoselectivities.