In order to construct the foamed concrete with low hygroscopicity meanwhile to recycle pyrolysis sludge ash（PSA） effectively， the reasonable dosage of PSA， sodium methyl-silanol（SMS） and nano-SiO₂ was studied by using orthogonal design in which the apparent dry density， thermal conductivity and 28 d compressive strength were set as the measuring indicators. Then the physical properties of the foamed concrete with optimal proportion were investigated. The results show that， if setting the apparent dry density and thermal conductivity as the measuring indicators， the reasonable dosage of PSA， SMS and nano-SiO₂ are 30%， 1.5% and 3.0%， respectively. If taking the 28 d compressive strength as the measuring indicator， the reasonable dosage of these three raw materials are 10%， 0.5% and 3.0% respectively. In addition， the results show the addition of 1.5% SMS reduces the hygroscopicity of the foamed concrete by 90.6%-95.5% as compared to the control one without SMS. The thermal conductivity， density and strength of the foamed concrete with optimal proportion meet the basic requirements of FC5 self-insulating blocks.