An -locally repairable code (LRC) is called a Singleton-optimal LRC if it achieves the Singleton-type bound. Analogous to the classical MDS conjecture, the maximal length problem of Singleton-optimal LRCs has attracted a lot of attention in recent years. In this paper, we give an improved upper bound for the length of q-ary Singleton-optimal LRCs with disjoint repair groups such that based on the parity-check matrix approach. In particular, for any Singleton-optimal -LRCs, we show that: 1) , when and with ; 2) , when and for any . Furthermore, we establish equivalent connections between the existence of Singleton-optimal -LRCs for and with disjoint repair groups and some subsets of lines in finite projective space with certain properties. Consequently, we prove that the length of q-ary Singleton-optimal LRCs with minimum distance and locality is upper bounded by . We construct Singleton-optimal -LRC with disjoint repair groups such that and determine the exact value of the maximum code length for some specific q. We also prove the existence of -Singleton-optimal LRCs for .