•The fouling inhibition characteristics of an electromagnetic field is studied.•The electromagnetic field inhibits the deposition of CaCO3 fouling.•The electromagnetic field promotes CaCO3 formation in aragonite.•An evaluation index is proposed to define the fouling inhibition effect.•The fouling inhibition characteristics presents an extremum feature. Forced convective heat transfer and CaCO3 deposition experiments were performed to investigate fouling inhibition characteristics of a variable frequency electromagnetic field on the CaCO3 fouling of a heat transfer surface and three indicators, namely, fouling resistance, conductivity, and scanning electron microscope (SEM), were analyzed. Experimental results indicated that the variable frequency electromagnetic field could effectively inhibit the CaCO3 fouling deposition on the heat transfer surface and slow down the growth rate of CaCO3 fouling by influencing aqueous solution and fouling forming ions. Moreover, the variable frequency electromagnetic field could reduce the conductivity of the test fluid, increase both the total precipitation and the bulk precipitation, and promote the CaCO3 crystallization in aragonite rather than calcite to form muddy soft fouling that was readily washed off by the fluid flow. An evaluation model was proposed to better define the fouling inhibition effect of the variable frequency electromagnetic field in the whole period of CaCO3 deposition. The fouling inhibition characteristics of the variable frequency electromagnetic field presented an extremum feature. The minimum fouling resistance was 1.1 × 10−4 m2⋅K/W obtained at 1 kHz, during which the fouling inhibition rate was 64.7% and the average growth rate of CaCO3 fouling dramatically decreased by 61.0% compared with that of the blank test. This work can provide sound theoretical guidance for future application of electromagnetic fouling inhibition technology in the heat transfer enhancement.