The critical content of the substitution element M for Fe in LaFe_11.52-xM_xSi_1.48 (M=Ni/Cr) alloys was studied. When the content exceeds the critical content of the element M, the nature of magnetic transition of the alloy changes from first order to second. And the relationship between phase formation and magnetic properties in LaFe_11.52-xM_xSi_1.48 (M=Ni/Cr) alloys near critical value of Ni/Cr has been discussed sufficiently. The critical content of Ni element is x=0.08 and that of Cr is x=0.25. With the increase of Cr content, the α-Fe grows up obviously in LaFe_11.52-xCr_xSi_1.48 alloys. The impact of partial substitution of Ni for Fe is superior to that of Cr on the magnetic properties of 1:13 phase. The substitution elements and the content of those elements have different effects on the nature of magnetic transition and magnetic entropy changes of LaFe_11.52-xM_xSi_1.48 alloys.