FeSiAl magnetically soft alloy hollow microspheres were prepared by self-reactive quenching technology based on Fe+Si+Al and Fe+Si+Al+KNO3 reactive systems, respectively, in order to obtain absorbents with light weight, low frequency and high efficiency. Effect of reaction heat-releasing on their density, morphology, phase and microwave absorption properties in low frequency band was studied by SEM, XRD and vector network analyzer. The results show that, chemical reactions can not happen without KNO3. In addition, the quenching products consist of spherical particles, near-spherical particles, irregular particles and lamellar particles. The phase components are Fe, Si, Al and Fe0.9Si0.1. FeSiAl alloys are not formed and their microwave absorption properties do not exist basically. After adding KNO3, the quenching products mainly consist of hollow microspheres with uniform particle size distribution. The phases are made up of Fe3Si0.5Al0.5, Fe3Si0.7Al0.3, Fe0.9Si0.1 and Fe. FeSiAl alloys are obtained. The minimum reflectivity of the absorbent sample at 5 mm is -22.1 dB, and the corresponding frequency is 7.0 GHz. The effective absorption frequency band lower than -10 dB is 5.3-8.5 GHz, with the bandwidth of 3.2 GHz. The high heat-releasing and large amount of gas from the decomposition of KNO3 makes the reaction temperature be higher. In addition, the target products are obtained and then melt. Moreover, the spherical rate and hollow rate are both improved. These are the reasons for obvious improvement of microwave absorption properties in low frequency. Compared with flake FeSiAl magnetically soft alloy prepared by mechanical alloying method, FeSiAl hollow microspheres prepared by self-reactive quenching technology have a broader absorption frequency band in low frequency. In addition, the density of the material has decreased by 38 %. So the characteristics of new absorbents in low frequency including “thin, broad, light and strong”are effectively realized.