One of the crucial issues in modern ash chemistry is the realization of efficient and clean coal conversion.Industrially, large-scale coal gasification technology is well known as the foundation to improve the atom economy. In practice, the coal ash fusibility is a critical factor to determine steady operation standards of the gasifier, which is also the significant criterion to coal species selection for gasification. Since coal behaviors are resultant from various evolutions in different scales, the multi-scale understanding of the ash chemistry is of significance to guide the fusibility adjustment for coal gasification. Considering important roles of molecular simulation in exploring ash chemistry, this paper reviews the recent studies and developments on modeling of molecular systems for fusibility related ash chemistry for the first time. The discussions are emphasized on those performed by quantum mechanics and molecular mechanics, the two major simulation methods for micro-scopic systems, which may provide various insights into fusibility mechanism. This review article is expected to present comprehensive information for recent molecular simulations of coal chemistry so that new clues to find strategies controlling the ash fusion behavior can be obtained.