These numbers don't mean anything without some context. What is the assay? How are the measurements made? How much enzyme was present in each case? By what means is the enzyme immobilized (covalent, noncovalent)?

They reflect enzyme activity measurements for 3 and 4 mins (spectrophotometric assay). The measurements were performed by taking 300ul of enzyme + 300 ul of magnetic nanoparticles and incubated for 3 h at room temperature. The immobilization method was noncovalent adsorption. But the adsorbed enzyme (precipitate) has weak activity compared to soluble one...

It isn't surprising that the nanoparticle-adsorbed enzyme has lower activity than the enzyme in solution. In fact, this is precisely the mechanism that was proposed about 20 years ago by McGovern and Shoichet to explain why some chemical compounds behave as non-specific enzyme inhibitors. They showed that some compounds form nanoparticles that remain suspended in solution, and adsorption of the enzyme to these nanoparticles inhibits the enzyme. Presumably, this is because adsorption causes a structural change in the enzyme, or blocks the active site, or prevents the enzyme from undergoing a necessary conformational change involved in catalysis.

In order to avoid this sort of nonspecific inhibition, you should look for a way to immobilize the enzyme by a flexible linker that allows it freedom of movement.

It may also be that very little enzyme is actually adsorbed. You should try to quantify how much of the added protein is removed from the enzyme solution by the nanoparticles by measuring the protein concentration in the solution before and after the immobilization protocol, once the nanoparticles are removed.