The critical heat flux(CHF) is studied in a confined space both experimentally and analytically. The confined space is consisted of two horizontal surfaces with the lower surface heated and the upper mesh screen. The observation shows that the behaviors of the vapor mushroom over the mesh screen have not significant differences with that in the conventional pool boiling. However, the CHF reduces with the decrease of the space gap when it is narrower than someone critical value. The theoretical analyses show that the reduced CHF is related to the earlier dryout of macrolayer during the period of vapor mushroom because the initial thickness of the macrolayer reduces with the decrease of the space gap. However, for larger enough space, the CHF is reached due to the crisis of the local heat transfer by the microlayer model but not the macrolayer dryout. Analyses show that the heat transfer crisis(CHF) causedby the macrolayer dryout occurs only in advance of that by the microlyer model and the conventional macrolayer dryout model is a special case of the microlayer model. Theoretic prediction of CHF agrees reasonable well with experimental data.