Numerical simulation on low-speed magnetic flux leakage inspection to heat exchanger pipes is described. Statistics suggest that 90% of defects in ferromagnetic heat exchanger pipes occur under the retaining plate or nearby, making it a key monitoring area. This paper illustrates influence of different velocities, defect depth and position on the characteristic of flux leakage signals. Result of the numerical simulation provides theoretical principle for application of Magnetic flux leakage (MFL) technique to inspection of heat exchanger pipes. It possesses great significance to enhance the possibility for defect inspection as well.