The presence of a metastable fluid-fluid critical point significantly reduces the free-energy barrier for crystal nucleation, thereby increasing the nucleation rate by many orders of magnitude. This effect is crucial for promoting protein crystallization, as the success of crystallization depends on the physical conditions of the initial solution. The second virial coefficient $ B_2 $ of the protein solution is a key factor, with values indicating the likelihood of crystallization. For small negative $ B_2 $, crystallization is more likely, while large positive values prevent crystallization. The phase diagram of proteins shows that short-range attractive interactions lead to a fluid-fluid critical point, which influences the nucleation process. Simulations show that near this critical point, the free-energy barrier is significantly reduced, leading to enhanced nucleation. The nucleation rate increases by a factor of $ 10^{13} $ near the critical point, due to lower interfacial free energy. This mechanism is general and applicable to various systems, including compact macromolecules and quasi-two-dimensional systems. The study highlights the importance of critical density fluctuations in facilitating the formation of ordered structures.The presence of a metastable fluid-fluid critical point significantly reduces the free-energy barrier for crystal nucleation, thereby increasing the nucleation rate by many orders of magnitude. This effect is crucial for promoting protein crystallization, as the success of crystallization depends on the physical conditions of the initial solution. The second virial coefficient $ B_2 $ of the protein solution is a key factor, with values indicating the likelihood of crystallization. For small negative $ B_2 $, crystallization is more likely, while large positive values prevent crystallization. The phase diagram of proteins shows that short-range attractive interactions lead to a fluid-fluid critical point, which influences the nucleation process. Simulations show that near this critical point, the free-energy barrier is significantly reduced, leading to enhanced nucleation. The nucleation rate increases by a factor of $ 10^{13} $ near the critical point, due to lower interfacial free energy. This mechanism is general and applicable to various systems, including compact macromolecules and quasi-two-dimensional systems. The study highlights the importance of critical density fluctuations in facilitating the formation of ordered structures.