It is found that the large magnetostrain can be obtained from the magnetic-field induced first-order transition in some magnetic-phase alloys. The magnetostrain is generated from the lattice parameter alteration during the transition, which is quite different from that in conventional magnetostrictive alloys, such as Terfenol-D. However, due to the first-order nature of the transition, the observed magnetostrain is accompanied by obvious irreversibility. To overcome this drawback, we indicate that large reversible magnetostriction can be fulfilled in MnCoSi-based alloys at room temperature. By tuning tricritical behavior, the temperature of tricritical point and the critical driving field can be reduced. Based on this improvement, the room-temperature magnetostriction is generated from low-field induced second-order transition in MnCoSi-based alloys, which is characterized as high reversibility. We also introduce the mechanism and method of tricritical-behavior-tuning. The magnetostrictive effect in MnCoSi-based alloys is comparable to Terfenol-D, indicating potential applications in sensor, actuator, magnetomechanical relays, etc.