There are certain nonlinear PDEs naturally arsing from the study of Frobenius manifolds, stability conditions, representation theory and so on. The PDEs are a multi-time dependent Hamiltonian system, and have a Lax representation as the isomonodromic deformation of a linear system. In this talk, we take the Rimann-Hilbert approach to study various (global) properties of the PDEs, including the long time hehaviors, boundary conditions, connection formula, algebraic solutions and so on.