Aiming at the problem that online detection of weld defects during laser welding of carbon steel thin plates is susceptible to vibration interference, and traditional non-destructive testing methods fail to achieve highly reliable real-time monitoring in confined detection spaces, a novel online weld detection method is proposed based on laser-excited ultrasonic waves and ultrasonic reception by dual electromagnetic acoustic transducer (EMAT). A finite element model for the 'one-transmit and two-receive' weld detection process of laser electromagnetic ultrasound is established. Taking 4 mm-thick carbon steel butt welds with 2 mm-diameter circular hole defects as the detection objects, the effects of parameters such as laser spot shape and size, number of turns and spacing of EMAT coils, and distance between the excitation point and receiving points on the ultrasonic transmission coefficient are analyzed, followed by experimental verification. Univariate analysis indicates that the excitation efficiency of a sheet-shaped spot is superior to that of a circular spot, and higher defect identification sensitivity is achieved when the coil has 4 turns with a turn spacing of 2.0 mm, and the excitation point is positioned at distances of 11 and 36.4 mm from the two receiving EMATs, respectively. On this basis, the optimal parameter combination is obtained through orthogonal experimental design, and a mobile laser-electromagnetic ultrasonic testing system is established, and experiments are conducted on butt welds containing circular holes of different diameters. Experimental results show that for the optimized laser electromagnetic acoustic transducer (Laser-EMAT) system, the attenuation amplitudes of the transmission coefficient for Φ3 mm and Φ2 mm circular holes are 1.53 times and 3.96 times those before optimization, respectively, indicating higher defect detection sensitivity. For example, the deviation coefficient caused by defects is 1.91 times and 4.98 times that before optimization, realizing more reliable detection. For instance, the signal variation coefficient in the defect-free area decreases from 6.77% to 5.41%. The optimized Laser-EMAT system yields a deviation degree of 0.62 and a 3.36% reduction in the transmission coefficient for a Φ1 mm-diameter circular hole. Although these results do not meet the requirements for reliable detection, the system eliminates the negative fluctuations corresponding to the worst parameter combination and significantly improves the detection sensitivity and reliability.