Isolated attosecond pulses from an X-ray free-electron laser are in high demand for attosecond science, which enables the probing of electron dynamics by X-ray nonlinear spectroscopy and single-particle imaging. The aim of this thesis is to simulate attosecond pulse trains at the Athos beamline of the SwissFEL facility. The research project investigates the possibility of generating the shortest pulse duration by incorporating several advanced configurations at Athos, namely the modular undulator, the magnetic chicane after each module, and the transverse gradient undulator configuration. The simulation results confirm the effectiveness of these configurations in reducing the pulse duration. For a radiation wavelength of 1 nm (1240 eV), the slicing method, which is basically an energy chirp-based taper on a modular undulator, is able to produce a pulse train with an average duration of 200 attoseconds. The mode-locking method, realised by delay control through magnetic chicanes after each module, further reduced the average pulse duration to less than 100 attoseconds, exceeding the cooperation length limit. In addition, we explored the possibility of applying a linear taper within the module by incorporating a rotated transverse gradient undulator for each module. For the 1 nm radiation wavelength, a slight linear taper within the undulator module can reduce the pulse duration to 85 attoseconds. For the 4 nm (310 eV) wavelength, the slippage effect is so strong that the pulse duration first decreases and then increases within the undulator module. To solve this problem, we have proposed using a strong undulator taper within the module. A 16 % difference taper range combined with the mode-locking method can generate a pulse train with an average duration of 110 attoseconds to the end of the module. It is important to note that the results presented in this work are primarily used to demonstrate the method's validity. Experimental results in the SwissFEL machine can differ significantly from the simulation due to many other effects. In addition to the simulation and presentation of simulation results, the thesis has briefly included the FEL theory and used the theory to analyse the simulation results, which makes the result more understanable and plausible. During the discussion of the slicing method, we presented the bunching evolution in the slicing method and suggested that the superradiance dynamics can be used to explain many simulation results in this chapter. In the chapter with TGU, we suggest that the reason for generating such short pulses is mainly because the large taper provides a large spectrum, which is beneficial for generating short pulses when doing mode-locking.