how to calculate suitable MCB(miniature circuit breaker) for SM2315D?

Continuous current at nominal power for that motor is 5 amps. You can look at the torque curve on our website for an SM23165D motor. It will show the power in wattage for different voltage supplies.

the calculation is not 170watt/48VDC= 3.54Amp?
I=power/volt
how you get the 5Amp?

The 5Amp came from the specification sheet for the SM23165D motor (which is the Class5 version of the older SM2315D) on the website.

so, it's mistake on graph shown?

The 48VDC is the supply voltage to the integrated voltage switch mode drive. That motor can actually pull more than 10Amps if the rotor is locked. The speed/torque chart came from the dynamometer. I'd go with the 5Amp for a good safety factor.