Phase-matchable (PM) capacity is an indispensable factor in evaluating the applicable prospect of a new nonlinear optical crystal. Obtaining an accurate and credible PM curve by the Kurtz-Perry method using a small single crystal is indeed crucial. Here, we reported a new borate NLO crystal Rb7SrY2(B5O10)(3) (I) and studied the influence of the arrangement of micro-[B5O10 ] groups on its birefringence. Interestingly, the synthesized polycrystalline powder and grinded single crystal display divergent PM and non-PM behavior, respectively. Combining theoretical calculations and analysis, I is demonstrated to be non-phase matchable for the second harmonic generation (SHG) process of a 1064 nm laser.