Cyanobacteria are photoautotrophic microorganisms which can be engineered to directly convert CO2 and water into biofuels and chemicals via photosynthesis using sunlight as energy. However, the product titers and rates are the main challenges that need to be overcome for industrial applications. Here we present systematic modular engineering of the cyanobacterium Synechocystis PCC 6803, enabling efficient biosynthesis of 1-butanol, an attractive commodity chemical and gasoline substitute. Through introducing and re-casting the 1-butanol biosynthetic pathway at the gene and enzyme levels, optimizing the 5 '-regions of expression units for tuning transcription and translation, rewiring the carbon flux and rewriting the photosynthetic central carbon metabolism to enhance the precursor supply, and performing process development, we were able to reach a cumulative 1-butanol titer of 4.8 g L-1 with a maximal rate of 302 mg L-1 day(-1) from the engineered Synechocystis. This represents the highest 1-butanol production from CO2 reported so far. Our multi-level modular strategy for high-level production of chemicals and advanced biofuels represents a blue-print for future systematic engineering in photosynthetic microorganisms.