The invention provides a molten iron 
slag collecting agent; expanded 
perlite is used as a substrate, the components and 
mass percentages (%) of the 
slag collecting agent are as follows : 69 ~ 76% of SiO2, 0.6 ~ 1.2% of FeO, 1.5 ~ 2.3% of CaO, 0.2 ~ 1.2% of Na2O, 1.0~1.8% of TiO2, 0.8 ~ 1.5% of MgO, 1.0 ~ 1.4% of K2O and 0.1 ~ 0.3% of P, the 
slag collecting agent also includes by 
mass percentage (%), 2 ~ 5% of an 
adhesive , 0.5 ~ 1.2% of a heat insulating agent, 0.3 ~ 2.0% of a deoxidizing agent, and the balance of Al2O3. The invention also provides a preparation method of the slag collecting agent, and the preparation method includes the following steps: (1) preparation of an expanded 
perlite sample, to be more specific, collecting and selecting blocky 
perlite ore of 5 to 8cm, crushing into granules by a 
crusher, screening and picking broken granules to 0.9 ~ 1.7mm, then adding a cosolvent solution to the screened granules for soaking for 10 ~ 15s, performing 
pre expansion treatment, draining away water, putting into an oven of 750DEG C, 
drying for 2 ~ 3min to remove the water adsorbed on the surface of granules to obtain a perlite sample with a micro-hole structure, and raising the temperature of the oven to 850 ~ 1100 DEG C for pre heating to expand pores in 
micro holes to obtain an expanded perlite sample with the micro-hole structure; (2) preparation of slag collecting agent 
slurry; and (3) preparation of the slag collecting agent. The slag collecting agent of the invention has the characteristics of low cost, low dispersion, good fluidity, and strong slag 
adhesive capacity.