The present invention relates to a method for preparing LiCa-LSX molecular sieves with mixed cations and applications thereof. The method comprises the following steps; preparing CaLSX molecular sieves; replacing all Na+ in NaLSX molecular sieves with Ca2+ by multiple times of exchange with a solution containing Ca2+; changing CaLSX molecular sieves to LiCa-LSX molecular sieves by exchange with Li+, wherein a solution containing Li+ is used, and the exchanged sample is directly filtered in vacuum without washing and dried at normal temperature; and activating and pre-treating the sample to obtain LiCa-LSX molecular sieves with mixed cations. The LiCa-LSX molecular sieves can be used as selective adsorbents for N2 and O2 in oxygen production processes by PSA/VPSA. In the present invention, the loss of Li+ caused by the replacement of Li+ with H+ because of hydrolysis is avoided, the cost is reduced and the treatment time is short.