We prepared extruded poly(vinyl alcohol). (PVA) foams through combination of the PVA thermoplastic processing technology and the supercritical carbon dioxide extrusion foaming technology. Boric acid (BA) was used as a cross-linking agent to enhance the PVA's melt strength and to improve the cell structure of its foam. Two different PVA/BA. cross-linking models were discussed, and we confirmed the BA-compound formation model in our system. By applying the in situ Fourier transform infrared spectroscopy and the melt viscosity analysis, we found that the PVA/BA cross linking structure was reversible. An increase in the BA increased the PVA's melt strength as well as its foam volume expansion ratio and cell density, so that fine and uniform cells formed. However, at a higher die temperature of >140 degrees C, the cross-linking between the PVA and BA was gradually broken, and this resulted in serious bubble collapse and a worsened cell structure.