In the thesis, we apply mean-field theory to discuss the critical phenomena in two-dimensional sandpile model. With the concept of mean-field theory, every site in the system is assumed to be independent of each other. Following the investigation of A. Vespignani and S. Zapperi[5], we have derived higher-order correction to various single-site densities of sand grains. In the calculation, we include the two-site correlation function between two nearest-neighbor sites, which contributes to the higher-order correction to the previous calculated densities. Last of all, we discuss the correction to critical exponents due to the higher-order correction of densities.