There are interactions among the three mutrients of plant，potassium(K)，calcium(Ca) and magnesium(Mg). Twenty two typical litchi (cv. Feizixiao) orchards and 8 typical longan (cv. Chuliang)，located in the main litchi and longan production regions of South China，were chosen in 2016～2019. A total of 193，186 and 159 litchi leaf samples were gathered at fruit swelling stage (FSS) of 2016，the terminal shoot maturing stage (TSMS) of 2016 and FFS of 2017，respectively. And，82，82，49 and 49 longan leaf samples were collected at TSMS of 2017，FSS of 2018，TSMS of 2018 and FSS of 2019，respectively. Foliar K，Ca and Mg concentrations in all samples were detected. Sufficiency range approach (SRA) was used to establish the diagnosis norms for interaction indexes of foliar K，Ca and Mg，which were characterized as K/Ca，K/Mg，K/(Ca+Mg) and Ca/Mg. The results showed that all the interaction indexes of foliar K，Ca and Mg significantly varied among trees at the same growth stage in the same year，between the same growth stages of different years and between various growth stages within the same growth cycle. The variation of the interaction indexes for the high yield population was obviously smaller than that for the whole population of both litchi and longan. However，significant difference was still observed for these indexes for various growth stages even in the same growth cycle (P<0.001) . It indicated that different diagnosis norms of interaction indexes were required for various growth stages of litchi and longan. According to SRA，while foliar K，Ca and Mg were within the appropriate ranges，the optimal values of foliar K/Ca， K/Mg，K/(Ca+Mg) and Ca/Mg were recommended as 3.1～4.2，4.1～4.9，1.8～2.2 and 1.15～1.54 at TSMS，and 0.51～0.92，1.63～2.81，0.40～0.73 and 2.62～3.66 at FFS for litchi，respectively. Similarly，those values were suggested as 1.55～2.06，8.9～10.2，1.32～1.69 and 5.1～6.2 at TSMS，and 0.46～0.61，5.3～7.5，0.42～0.56 and 11.1～12.8 at FFS for longan，respedively. The above diagnosis norms provide quantitative reference for evaluation of the balance of foliar K，Ca and Mg and their balanced application for both litchi and longan.