In this work, a new strategy was proposed to improve the foamability of poly(aryl ether ketone) (PAEK) based on novel long-chain branched PAEK (LCB-PAEK). To improve the resin melt strength, the amorphous LCB-PAEK was fabricated via a simple ternary copolymerization of phenolphthalein, 4,4'-difluorobenzophenone, and 1,1,1-tris (4-hydroxyphenyl)ethane. To regulate the extent of reaction and avoid crosslinking, 4-fluorobenzophenone was introduced as the end-capping reagent. The presence of long-chain branches were proved by gel permeation chromatography and rheological characteristics, including storage modulus and tensile viscosity. Afterwards, the prepared LCB-PAEK samples were foamed using supercritical carbon dioxide (ScCO2) as the blowing agent. The influence of branching content on foaming results was investigated in detail, and significant improvement in foamability was observed. By regulating the foaming parameters, the highest expansion ratio of 27.5 was achieved, which was almost ninefold higher than that of linear PAEK. In addition, the good mechanical properties indicated the promising applications of LCB-PAEK foams as high-performance structural foam.