Astragalus sinicus is a commonly used legume green manure that fixes atmospheric N2 and accumulates mineral nutrients and organic substances that are beneficial to soils and subsequent crops. However, little is known about genotypic variation in, and molecular mechanisms of, Pi (phosphate) uptake and storage in A. sinicus. We recorded the morphological responses of six A. sinicus cultivars from four regions of China to external Pi application and measured their Pi accumulation. We identified full-length transcripts of Pi-signaling and Pi-homeostasis regulators by sequencing and measured the expression level of these genes by qRT-PCR. The major components in Pi signaling and Pi homeostasis were largely conserved between A. sinicus and the model species rice and Arabidopsis. Different A. sinicus varieties responded differently to low-phosphorus (P) stress, and their Pi accumulation was positively correlated with the expression of vacuolar Pi influx gene (SYG1/PHO81/XPR1-MAJOR FACILITATOR SUPERFAMILY (SPX-MFS)-TYPE PROTEIN) AsSPX-MFS2 and negatively correlated with the expression of the vacuolar Pi efflux gene (VACUOLAR Pi EFFLUX TRANSPORTER) AsVPE1. We identified key Pi-signaling and Pi-homeostasis regulators in A. sinicus. The expression of vacuolar Pi transporter genes could be used as an index to select A. sinicus accessions with high Pi accumulation.