Stable and remarkable valley polarization effect is the key to utilizing valley degree of freedom in valleytronic devices. Recently, a novel collinear magnetic material known as altermagnet, which is different from ferromagnets and antiferromagnets, has attracted widespread attention. Theoretical studies have revealed that the monolayer altermagnet V₂Se₂O exhibits spin-valley locking induced by crystal symmetry rather than traditional time-reversal symmetry. Uniaxial strain can break the corresponding crystal symmetry, leading to a remarkable non-relativistic valley polarization effect. Therefore, beyond uniaxial strain, are there alternative strategies to break the crystal symmetry in altermagnets and achieve remarkable valley polarization? According to first-principles calculations and symmetry analysis, we reveal that valley polarization effect in monolayer V₂Se₂O altermagnet is correlated with the net magnetic moment between magnetic V atoms under uniaxial strain, thereby proposing two strategies for achieving giant valley polarization effect. Firstly, substituting one V atom in V₂Se₂O with Cr to construct a ferrimagnetic monolayer VCrSe₂O enhances the net magnetic moment between magnetic atoms, thereby realizing a giant valley polarization effect. Applying uniaxial strain along either the a-axis or b-axis significantly increases the value of valley polarization, which exhibits a nearly linear relationship with the net magnetic moments between the magnetic atoms. Secondly, constructing a van der Waals heterostructure composed of V₂Se₂O and α-SnO monolayers breaks mirror symmetry, thereby inducing a net magnetic moment, which in turn causes a remarkable valley polarization effect. Compressing the interlayer distance of the heterostructure can increase the net magnetic moment between V atoms, thereby enhancing the value of valley polarization to nearly 400 meV. This work reveals that valley polarization in monolayer altermagnet is correlated with the net magnetic moment between magnetic atoms. Finally, we propose two strategies to achieve giant valley polarization based on monolayer altermagnets, providing theoretical guidance for the potential applications of ferrimagnetic monolayers and altermagnet-based heterostructures in valleytronics.