  The Sandage-Loeb (SL) test is a unique method to probe dark energy in the
"redshift desert" of $2\lesssim z\lesssim 5$, and thus it provides an important
supplement to the other dark energy probes. Therefore, it is of great
importance to quantify how the future SL test data impact on the dark energy
constraints. To avoid the potential inconsistency in data, we use the
best-fitting model based on the other geometric measurements as the fiducial
model to produce 30 mock SL test data. The 10-yr, 20-yr, and 30-yr observations
of SL test are analyzed and compared in detail. We show that compared to the
current combined data of type Ia supernovae, baryon acoustic oscillation,
cosmic microwave background, and Hubble constant, the 30-yr observation of SL
test could improve the constraint on $\Omega_m$ by about $80%$ and the
constraint on $w$ by about $25%$. Furthermore, the SL test can also improve the
measurement of the possible direct interaction between dark energy and dark
matter. We show that the SL test 30-yr data could improve the constraint on
$\gamma$ by about $30%$ and $10%$ for the $Q=\gamma H\rho_c$ and $Q=\gamma
H\rho_{de}$ models, respectively.
