The invention discloses a wastewater treating agent for wastewater containing heavy metal. The wastewater treating agent is prepared from, by weight, 10-18 parts of nano-titania, 10-21 parts of dendrobium leaves, 45-73 parts of attapulgite, 10-17 parts of hydrochloric acid, 8-15 parts of ferric trichloride, 5-21 parts of polyacrylamide, 5-13 parts of dimethyl diallyl ammonium chloride, 1-5 parts of activated carbon powder, 3-14 parts of ethylenediamine tetraacetic acid disodium salt, 4-8 parts of tert-butylamine, 3-8 parts of hydroxymethyl starch, 1-5 parts of sodium silicate, 2-7 parts of sodium hydrosulfite, 4-16 parts of sodium hydroxide and 16-34 parts of water. The invention further discloses a preparing method of the wastewater treating agent. According to the prepared wastewater treating agent, the preparing process is simple, the raw material cost is low, and the raw material source is wide; the agent contains various active groups, the active groups and heavy metal can be subjected to a complexation reaction, a chelation reaction, an adsorption reaction, an exchange reaction and the like, and therefore heavy metal ions in wastewater can be removed. Thus, wastewater discharge can reach the standard, and remarkable economic benefits can be achieved.