Si nanopyramids have been suggested as one of the most promising Si nanostructures to realize high-efficient ultrathin solar cells or photodetectors due to their low surface area enhancement and outstanding ability to enhance light absorption. However, the present techniques to fabricate Si nanopyramids are either complex or expensive. In parallel, disordered nanostructures are believed to be extremely effective to realize broadband light trapping for solar cells. Here, a simple and cost-effective method is presented to form random Si nanopyramids based on an all-solution process, the mechanism behind which is the successful transfer of the generation site of bubbles from Si surface to the introduced Ag nanoparticles so that OH- can react with the entire Si surface to naturally form random and dense Si nucleus. For optical performance, it is experimentally demonstrated that the random Si nanopyramid textured ultrathin crystalline Si (c-Si) can achieve light trapping approaching the Lambertian limit. Importantly, it is revealed, by numerical calculations, that random Si nanopyramids outperform periodic ones on broadband light absorption due to more excited optical resonance modes. The finding provides a new opportunity to improve the performance of ultrathin c-Si solar cells with a simpler process and lower cost.