The present work focuses on studying the relationship between the time period of a polymer solution staying in the metastable region and the resulting porous structure after phase separation of the solution. We employed the technique of FrIR microscopy to determine the composition change in the PMMA/NMP (n-methylpyrrolidone) solution after its contact with water. By plotting the composition change on the ternary phase diagram of PMMA, NMP and water, and identifying the times that the composition path intersected with the binodal and the spinodal, we then determined the residence time of the solution in the metastable region (t(m)). For each polymer solution, we identified a critical residence time (t(mc)) that played a dominant role in determining the phase separation mechanism and the resulting porous structure: with t(m) less than t(mc), the corresponding structure was bi-continuous (typical structure resulted from spinodal decomposition); with t(m) greater than t(mc), the corresponding structure was cellular (typical structure from the mechanism of nucleation and growth). The results show that t(mc) depended on the polymer molecular weight and the polymer concentration in casting solution. And the dependences can be superimposed into one curve as the polymer concentration was normalized by the polymer chain entanglement concentration. With the relationship between t(mc) and the normalized polymer concentration and a simple model for the effective diffusivity of water in the polymer solution, we developed a model equation to estimate the positions in PMMA membranes where structure transition occurred from bi-continuous to cellular. Good agreement was obtained between the calculated transition positions and the experimentally determined ones.