The invention relates to a thin thermal battery spacer and a preparation method thereof. The preparation method is characterized in that the thermal battery spacer is prepared in the way that a matrix is impregnated in electrolyte molten salt, so that making molten salt electrolyte, which has extremely high probabilities of absorbing moisture and being dissolved, into the spacer becomes possible. The preparation method does not need a harsh operation environment, is good in repeatability and low in cost, and can be used for preparing spacers with different thicknesses; and meanwhile, the prepared thermal battery spacer is thinner and has the thickness capable of being adjusted within 150-350 [mu]m, so that the activation time of a battery is effectively shortened, and the high-power discharge capacity is effectively improved.